Add script to initiate a client release

Add a script to handle creating a new client release using yarn,
ensuring that the Git tag is signed. Unfortunately yarn's default is for
this setting to be turned off. Also unlike `npm version`, `yarn version`
does not support automatically incrementing the package version.

Fix login flow triggering popup blocker in Firefox and IE

Use the verb form for correctness in context and consistency with text
on the h web pages.

 - Use the term "auth" for consistency with the `authorize` method and use
   the verb "open" to clarify that the popup window is being both created
   and shown.

 - Clarify the relation of the `$window` parameter to the returned
   window.

hypothesis/client#603 broke the login popup window in Firefox and IE
because the call to `window.open` no longer happens in the same turn of
the event loop as the user's click on the "Login" link. It is therefore
no longer considered in FF to have happened "in response to a user
gesture".

This PR fixes the issue by separating creation and use of the popup
window into separate functions and moving creation to happen earlier, in
the same event loop turn as the "Login" button click.

Fixes #534

Now there are two separate links for requesting an invite to our public Slack workspace (slack.hypothes.is) and accessing the workspace once you already have an account (hypothesis-open.slack.com)

Allow whitelisted URL params in YouTube embeds

add support for `t`, `start` and `end` query
params for embedded YouTube videos.

Update reference to annotator CSS bundle.

`adder.js` contains a reference to the main CSS bundle generated from
src/styles/annotator which was not updated when the CSS bundle's name
was changed in f0b61e6a

Rename JS bundles to match corresponding source directory

Rename the "injector" and "app" bundles to "annotator" and "sidebar"
respectively to match the source directory which produces those bundles.

Rename entry point for annotator CSS bundle

Restructure sidebar styles in source tree

Rename entry points for annotator and sidebar bundles to 'index.js'

Update the docs for config options available to third party publishers.

Rename the entry point for the annotator CSS bundle so that it matches
the name of the source directory from which the bundle is compiled and
also the name of the corresponding JS bundle once that is renamed by
hypothesis/client#633

- Move the sidebar-specific styles into `styles/sidebar` and the
  component-specific styles into `styles/sidebar/components`. This mirrors
  the structure of the JS code in `src/`.

- Rename the entry point for the sidebar's style bundle from "app.scss"
  to "sidebar.scss" to make it more obvious what is in the generated
  "sidebar.css" bundle.

This move highlights some inappropriate dependencies between style
modules and an unclearly structured assortment of "stuff" shared by both
annotator and sidebar bundles in shared files in `src/styles`. There is
probably a fair amount of unused code in there as well. Cleaning that
up will happen in separate commits.

Rename the entry points to the more conventional 'index.js' for the
benefit of new visitors to the codebase and also for consistency across
the different bundles that make up the client.

Remove create account banner

The template no longer uses this property.

The text is potentially inappropriate in the context of
publisher-managed accounts where the account might not be free and the
user might not be able to create one themselves.

In the first party account context we agreed that the banner is a bit
redundant anyway. See
https://hypothes-is.slack.com/archives/C07NXBDNW/p1510080494000386

Disable close btn when the classic theme is on. Also fix errors with …

Disable close btn when the classic theme is on. Also fix errors with the preferences object being obsent from session state initially.

Remove unnecessary config in test.

Bundle the clean theme related config options into the theme = 'clean…