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/
@tilgovi wouldn't Web Platform currently be served by doing the following?

```python
config.override_asset('h:templates/client/auth.html', 'notes_server:templates/client/auth.html')
config.override_asset('h:templates/client/account.html', 'notes_server:templates/client/account.html')
```

I'm not sure how I feel about the macros approach, I can see how it keeps all of the overrides in a single file but on the other hand it means that we're going to end up with all these arbitrary bits of code extracted into "blocks" depending on who we're integrating with. This is going to lead to it becoming a dumping ground.

I think I'd rather remove this for the moment and explicitly replace these two templates on Web Platform. Then the next time we work on an integration that needs to replace content we design something a little more flexible like the CKAN example above.

Thoughts?