Use atomic git push during release process

Ensure that the release tag is only pushed to GitHub if the release
commit can also be pushed to master.

We are likely to change the release process soon to avoid the need to
push to master at all (see https://github.com/hypothesis/client/issues/810)
but in the meantime this prevents a scenario where:

 - A tag vX.Y.Z gets created
 - The package version is still at X.Y.(Z-1) on master
 - Subsequent builds of master fail because they try and fail to create
   a tag vX.Y.Z which already exists

Disable CoffeeScript support when building sidebar application

All of the modules and tests for the sidebar app that were written in
CoffeeScript have either been removed or converted to JS.

Disable CoffeeScript support when building the sidebar bundle and stop
searching for tests written in CoffeeScript in the src/shared/ and
src/sidebar directories.

Rewrite used parts of stream-filter.coffee in JS

Convert discovery.coffee to JS

Convert shared/discovery tests to JS

Add `make docs` and `make checkdocs`

Add postMessage config delivery example page

Convert viewFilter tests to JS

Deleted the "docs for how to build the docs" since this is now just a
simple `make docs`, as for all our projects.

`setMatchPolicyIncludeAny` no longer exists and is not needed since it
is the _only_ match policy.

This test case reproduces usage of `streamFilter` in
annotation-viewer-content, where the operator is "equals" not "exactly".

Remove support for real-time updates in the /stream view

It is not particularly clear from the naming of these modules what they
are for and how they relate to things the user can do in the
application. Add some comments to clarify this.

Convert the tests for the `viewFilter` class to JS. This is mostly a
straightforward conversion except that `viewFilter.fields` has been made
a private variable and the direct tests for ithave been removed, since the
contents of this field were an implementation detail of the class.

This is straight manual conversion of tests for the `Discovery` class to
JS.

Add notes to `Discovery` class docs that provide an overview of the
current discovery protocol.

Adjust spinner positioning and convert to component

We need to fix these, but for now I'm just making a note about the
issue.

 - Convert `Discovery` class to JS and improve the documentation. This
   is a manual conversion.

 - Rename private `_generateToken` method to `generateToken` because it
   is currently used by cross-frame.coffee. It should really be a
   utility method or cross-frame.coffee could use a different method to
   generate the ID.

Port stream-filter.coffee to JS and add tests, but only for the
functionality that we actually still use in the app. Following the
removal of support for real-time updates in the stream, the websocket is
only configured to filter by URI, annotation ID or reply IDs
("references") in the sidebar and standalone annotation viewer.

Set a useful title on the sidebar iframe

This was untested code, some of the last CoffeeScript in the sidebar
app, that supports a feature which gets little use.

The stream itself will still be available at https://hypothes.is/stream
but visitors will need to refresh the page to see updates.

This went unnoticed because it turns out that `yarn version` does not
actually run the postversion hook if `--no-git-tag-version` is
specified. See https://github.com/yarnpkg/yarn/issues/5358

Deploy QA release of client as part of master branch builds

This makes it easier to identify the purpose of the frame when
navigating the page with assistive technology.

Fixes https://github.com/hypothesis/product-backlog/issues/784

 - Remove a semantically incorrect `<button>` wrapping the loading
   spinner and a redundant wrapping `<span>`.
 - Vertically center the spinner by adding a wrapper `<div>` in the
   spinner component itself, so that there is less of a visual shift
   when switching between displaying the loading indicator and the
   search bar.
 - Add a meaningful title attribute to the spinner. Properly indicating
   when the sidebar's content is changing/being loaded to ATs requires
   additional work which is not tackled here.

This is the last remaining directive which should be a component.

The short-lived max-age value set previously was overridden by the
higher value of 1800 (30 mins) set in CloudFlare's "Browser Cache TTL"
setting under Page Rules for cdn.hypothes.is.

Resolve this by simply disabling caching of the entry point for qa
deployments. This means that https://cdn.hypothes.is/hypothesis@qa will
always return the boot script for the latest client QA release.

Add a new build stage to Jenkins CI builds of the master branch which
deploys a "QA" build to S3. This QA build:

 - Has a pre-release version number of the form `X.Y.Z-{commit hash}`
 - Has a stable URL at https://cdn.hypothes.is/hypothesis@qa
   (as opposed to https://cdn.hypothes.is/hypothesis for the prod build)
 - Loads the sidebar app from qa-h (https://qa.hypothes.is/app.html)
 - Does not update the changelog or create a Git tag or GitHub release

This build is currently not deployed to npm at all, but we might need to
do that in future if we were to create a QA deployment of the browser
extension with it.

---

 - Add a QA deploy step to the Jenkins build.
 - Skip GitHub release creation and changelog updates for pre-release
   versions used for QA deployments.
 - Remove the preversion.sh script, which is no longer needed since the
   builds always run in Jenkins and not on a developer's system.
 - Support specifying the npm tag to check in `wait-for-npm-release.sh`,
   though this is not used in the final version of this commit.

Replace the generic Docker-based s3-npm-publish tool with a JS script in the
client repo which is tailored more specifically to the needs of client
deployments:

 - Avoid creating the `hypothesis@X.Y` and `hypothesis@X` aliases which we
   never used.

 - Support uploading a copy of the entry point under a version-indepenent
   alias, regardless of whether this is a pre-release or not.

   This will enable creating a stable URL which points to the current QA
   version of the client.

 - Deploy the package directly from the current working directory. This saves
   the need to wait for publication to npm to complete, or the need to
   actually publish the package to npm at all in the case of the QA
   release.