app.coffee failed to initialize Raven because it looked for
config info in window.RAVEN_CONFIG instead of the #hypothesis-settings
<script> tag.

This commit makes the web service expose the Raven config info
to the site and the application in the same way, using JSON <script>
tags which are compatible with the strict CSP policy used by
the Chrome extension.

 * Expose and consume config info in the same way on
   the site and in the app.

 * Refactor settings.js to make it independent of Angular, for
   use on the site.

Fixes #2938