Update server side templates to use Jinja

Dokku: https://jinja.dokku.hypothes.is

Jinja[1] is a far more front-end friendly tempting system that is similar to the language that Django[2] uses, (it also has analogs in PHP[3] and Node[4]) so it's familiar to a lot of people.

This pull request...

* Swaps out chameleon for Jinja across the board.
* Moves angular templates, layouts and helpers into their own directories.
* Renames embed.txt to embed.js to avoid confusion around the content type.
* Includes a custom Jinja extension for including raw content (used for including Angular templates).

[1] http://jinja.pocoo.org/docs/dev/
[2] https://docs.djangoproject.com/en/dev/topics/templates/
[3] http://twig.sensiolabs.org
[4] https://mozilla.github.io/nunjucks/
@hypothesis/developers any idea what this is doing? I couldn't find it used anywhere...