Report form level errors

When we call `form.validate()` and the validation fails, the raised
exception is a `deform.ValidationFailure`. This object has an `error`
attribute which represents the underlying validation error object for
the entire form.

Previously, if the validation failure were a result of top-level
validation errors (such as the fact that an unactivated user is trying
to log in), this error would be lost, because we only reported errors
for the forms fields, or "children".

This commit changes `h.accounts.views.validate_form` so that it converts
the entire `colander.Invalid` object into a dictionary using its
`asdict()` instance method. By doing this, we get two immediate
benefits:

- top-level validation errors are reported in the '' (empty string)
  field
- we avoid the need to aggregate form field errors by hand in ajax_form

In addition, we need to deal with this case on the frontend, so this
commit also changes the formRespond directive so that if no overall
"reason" is provided for failure, then an empty-string member of the
"errors" object can set the overall validation status (and error
message) for the form.

Refactor search

Ask for "Username or email address" when logging in

Fully isolate os.environ during config testing

Dehorusify the AuthController

Keep app css and inject css separate

Structure is good. Bonus fewer false dependencies on asset rebuilds
for the app.

Dehorusify ProfileController

Fix display regression on help/first-run pages

Let users change their email addresses

1. Go to the Change Your Email Address form
2. Type in a new email address (twice) and your password
3. Select part of the text showing your current email address with the
   cursor
4. Click Update

=> The display of your current email address is updated to a nonsense
value.

This commit fixes the issue, your email address will be displayed
correctly.

Move the user's current email address out of the placeholder attribute
of the new email address <input> field. Just put it in a <p> instead.
This is much better for usability.

Quite a lot of repetition in these tests, some shared helper functions needed.

Begin adding frontend tests for the new AccountController code that
deals with the new form for changing the logged-in user's email address.

I've done this as a new top-level describe() with a new way of writing
Angular controller tests, rather than adding to the stuff already in
account-controller-test.coffee.

The aims of this way of writing tests is to make the tests easier to understand
and modify (either to fix a broken test or to add new ones) by:

- Reducing special Angular knowledge needed to understand the tests

- Reducing globals (both global variables and beforeEach() functions) used in
  tests.

  These increase the "travel" needed to understand a given test method because
  you have to read the whole test module to understand everything that happens
  before the test runs and what all the global variables the test uses are.

  The aim is that reading only the test function (and not the rest of the code
  in the file) should be enough to understand the test.

  Similarly I shouldn't have to read and understand the whole test file to add
  a new test.

  Globals and beforeEach()'s also tie the tests together. For example if a
  bunch of stub services and a controller are created in beforeEach() functions
  then any new tests added to the file inherit this stubbing behaviour even
  though they may not want it or may need something else. This again makes
  changing or adding just one test harder - need to understand the whole file,
  changes made for one test may break other tests.

Implementation notes:

- Used a new top-level describe(), didn't want to inherit all the globals,
  beforeEach()'s and stubbing of the existing one.

- Because we now have two top-level describe()s had to use a try ... catch in
  the second one to avoid creating the "h" Angular module twice.

- Tried to decouple the tests from Angular as much as possible, reduce the
  amount of Angular knowledge (especially about providing and injecting
  dependencies) needed to understand the tests.

  Angular's $provide isn't used at all, and inject() (which I don't think can
  be removed completely) is contained in two helper functions.

- Rather than a beforeEach() and global variables we use a controller()
  function that creates the AccountController and stubs for the services it
  depends on and returns an object of all of them.

  Tests can then call controller() and use destructuring assignment to get only
  the bits they need.

  Every variable used in a test method is defined in the test method
  (sometimes by calling a helper function and getting a return value).

- controller() by default uses minimal stubbing, but the caller can optionally
  pass in their own object for each of the stubbed dependencies.

Fixes #174.

account.html:

- Add a new "Change Your Email Address" form

accounts/views.py:

- Add email to the user dict returned at /app?__formid__=profile (or
  session.profile() in Angular)

- When resetting the form after a successful submission, put the new (or
  unchanged) email address into the user dict returned in the response.
  AccountController needs this (see below).

account-controller.coffee:

- Add $scope.email (the user's current email address), to use for placeholder
  text in the email form field.

- After a successful form submission set $scope.email to the one from the
  response. This sets the placeholder text in the email field to the new value,
  instead of continuing to show the old email address as placeholder.

- Add changeEmail() method to receive email form submissions.

Display KaTeX parse error upon faliure to parse

Added mathjax fallback to inline rendering

This change introduces a new top-level stylesheet for the help page.
This will hopefully make it easier to tell where styles affecting the
help page come from, and minimise the number of times that changes to
unrelated CSS will alter the display of the help page.

Overriding properties on the body in the otherwise component-specific
`topbar.scss` is a cause for confusion. This resulted in the styling for
the help page being broken when topbar was bundled into `app.scss`.

Add URL params to /stream.atom

This reverts commit f6787df785c8c8e2bdbf4d7b65a9176adad64d19, reversing
changes made to 29d975367171b8d4e69941c3f0e5d40ddfc98ddc.

I'm afraid I've had to revert this:

- it introduced a crashing bug on master (see
  https://github.com/hypothesis/h/pull/2131#discussion_r30692024)
- feature flags are no longer accessible by view functions (the
  configurator object is not available from the request), thus this
  blocks #2131