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/
There's a pretty nice extension for the Django templating system (which is superficially very similar to Jinja2) that allows for exactly this pattern: [overextends](https://github.com/stephenmcd/django-overextends).

Namely, if the parent application provides a template, say `index.html`, which defines some blocks:

```jinja
{% block head %}
<link rel="stylesheet" href="main.css">
{% endblock %}

{% block body %}
<p>Hello, world!</p>
{% block body %}
```

Then a child application could simply drop a template with the same name (`index.html`) into its template path containing:

```jinja
{% overextends "index.html" %}
{% block head %}
{{ block.super }}
<link rel="stylesheet" href="custom.css"
{% endblock %}
```
and the result is what you want, I think. The head block will contain both stylesheets and the body block will be inherited from the parent template.

I can't immediately see something similar for Jinja2 but I can't imagine it would be immensely difficult to port django-overextends.