Move worker count to environment variable

There are problems with races creating databases on startup. We could
stop creating them automatically, but it's mighty convenient and its
not easy to run the db creation script otherwise. For instance, in
a dokku the docker links are not available during the buildstep.
Sort orphan placeholders to bottom of stream
Merge pull request #1623 from hypothesis/stream-orphan-sort

Sort orphan placeholders to bottom of stream
Merge pull request #1622 from hypothesis/simplify-highlights

Simplify highlight visibility toggling.
Handle undefined data on 'auth' event for AuthAppController

When switching tabs on our /login route, the scope.on 'destroy' is
fired in the AuthController which emits an auth event with undefined
data value.

Our AuthAppController was trying to check the data.userid for this
undefined value. We only need to do that when data is undefined.

Fixes #1627
Remove elide filter

This was only used by the old (non-unified) stream template.
Add a config.js file to the Chrome extension

This allows the Chrome extension to be configured with hypothesisConfig.
Also moved this file and the destroy.js file into an extension
subdirectory to avoid confusion with the main app files.
Show highlights when showHighlights option is provided
Add unit tests to verify showHighlights option
Include the hypothesisConfig in the bookmarklet