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/
I should clarify that the original template doesn't have to be copied wholesale, the extension one just overrides the blocks that it needs to, the rest automatically falls back to the default content.