Mock req when converting wiki to HTML

Some parts of the conversion code tries to use permissions or other
parts of the request, which aren’t always present (for example when we
convert wiki text on the console)
Visual improvements of comments display

* Comments now start from the beginning of the row – helps with recognizing
when comments start
* Lighter colors
* Paddings and margins have better harmony
* Highlighted comments are using less harsh colors than yellow
Move page-level comments to the bottom

If they’re at the top and somebody goes to the page just to look at the
file and not at the comments, a huge discussion at the top can be very
distracting.

We assume that people will go to pages more often to see the
files/diffs/commits than to look at comments.
Add source maps for Backbone and Underscore
Remove the line-comment template

Wasn’t used anywhere
Added (minimal) UI for subscriptions

Added `SubscriptionModule` to handle subscriptions views:

 * Inject a (disabled) button into `changeset`, `browser`, and
 `attachment` pages.
 * Handle `GET`, `POST`, and `PUT` requests for subscriptions, accepting
 and returning only JSON content

Added `Subscription.for_request()` to get or create a subscription from
a request.

Added Backbone `Subscription` model and `SubscriptionView` view to
handle UI interaction.

Modified email template to let users know how to unsubscribe.
Replaced 404 response with 204 (empty)

For subscriptions that do not yet exist, return a 204 `NO CONTENT`
response instead of a 404 `NOT FOUND`. Backbone handles empty reponses
correctly, and the 204 code will not trigger a jQuery `ajaxError` event.