Make failed API calls reject with an Error

Previously failed API calls from the `store` module were rejected with a
{ status, statusText, data, headers } response object.

This had several problems:

 * This violated the convention that promises should be rejected with
   Error or Error-like objects, so callers could not rely on the result
   having a `message` property.

 * It was up to each of the callers to translate the result into
   a message suitable for display, which is likely to lead to
   inconsistency.

 * Testing the translation of different kinds of failures into
   messages was mixed up in the `<annotation>` component tests.

This commit moves the translation of $http responses to `Error` objects
into the API client (`store`) module and adds tests.

Work around `getComputedStyle` bug in Firefox

Fix inconsistent component exports

This could return `null` in Firefox when the sidebar iframe is hidden.

Currently the app only ever has one `<thread-list>` instance, so
we can simplify the logic that gets the scroll root to just query for a
hard-coded selector.

`<thread-list>` uses `getComputedStyle` to find the nearest scrollable
ancestor element which it needs to know in order to scroll specific
annotations into view and determine which annotations are visible.

In Firefox `getComputedStyle` may return `null` if the iframe is hidden
- which may be the case when this code is called in the sidebar.  This
is considered a bug upstream. Work around it in the meantime by adding a
specific class (`js-scrollable`) to the scrollable element and testing
for that as a fallback.

Fixes #341

This fixes a small inconsistency in that most component modules exported
the object defining the component directly, whereas others exported an
object with the component as a property. There is no good reason to do
the latter.

Use Sphinx environment variable cross-references

Remove unused vm.feature variable

As unfortunate as the one-letter name is we agreed to just call it h and
not the service, and this is how it's used elsewhere in the docs.

This generates warnings if cross-references (for example to environment
variables) are broken.

When mentioning an environment variable in Sphinx docs do this:

```
:envvar:`FOO`
```

This will:

1. Render the envvar mention as a `<code class="std-envvar">`
   (among other CSS classes), this is then styled specially by the
   Sphinx theme

2. Automatically hyperlink it to the env var's definition on the
   env vars page

3. Generate a warning if an envvar with that name isn't defined
   (if the -n arg tp sphinx-build is used)

This is for consistency with our variable names and, for example,
environment variable names which use `HOSTNAME` rather than `HOST_NAME`.

I want this file to be in rst so that I can use some rst features in it
in future commits. The main changes in order to translate from markdown
to rst are:

* Inline `<code>` requires double-backticks not single

* These titles don't work:

  ```
  # Heading

  ## Subheading
  ```

  They should be:

  ```
  Heading
  =======

  Subheading
  ----------
  ```

* To get an auto-numbered list, repeatedly using 1 doesn't work:

  ```
  1. First item
  1. Second item
  1. Third item
  ```

  Instead use `#`:

  ```
  #. First item
  #. Second item
  #. Third item
  ```

* Paragraphs starting with `**Tip**:` (and with the contents in italics)
  now become proper tip blocks:

  ```
  .. tip::

     If the output of ``hostname`` does not include a ``.home`` or ``.local``
     suffix, you may need to append ``.local`` to get a host name that is accessible from
     other devices on the network. If you have problems using the host name, try
     using the IP address instead.
  ```

  Sphinx renders these boxes as `<div class="admonition tip">` and
  they're styled by your Sphinx theme (as opposed to hardcoding the tip
  is in bold and the body is italics but otherwise it's just a `<p>`).

* Code blocks are like this:

  ```
  .. code-block:: sh

     First line of code
     Second line of code
  ```

Remove unused controller properties

Remove obsolete `ng-cloak` directives

Add Slack link to README

Invite users and potential contributors to join us on Slack as well as
IRC.

Add docs explaining how to test the client on a real mobile device

Use angle brackets to indicate that HOSTNAME is a placeholder rather
than a dollar-sign to avoid confusion with the `$HOSTNAME` env var.

 * Add note about possible need to add `.local` or `.home` suffix or use
   IP address instead of hostname.
 * Use title case for heading.

Merge pull request #333 from hypothesis/fix-annotating-sites-with-broken-Function.bind-polyfills-take-3

Fix annotating sites with broken function.bind polyfills take 3

Add a lint rule to ensure that new developers do not use `bind` in
annotator/ code. This only applies to first-party code. Third-party code
used in the annotation layer is still at-risk of being broken by old
`bind` polyfills.

Don't call the parent browsing environment's Function.bind() function
from the src/annotator code.

Some parent pages modify Function.protoype.bind(), for example replacing
it with their own broken bind() polyfill, and this causes our code to
crash.

Work around this by just not using bind().

Fixes #245

Translate annotation-sync.coffee to JavaScript

Use new endpoint for creating an annotation flag

This function has no tests and I don't know what it does but sometimes
`undefined` is passed to it as the `annotations` arg and it crashes, so
fix that.