Goodbye mustache.js, hello GRMustache.

The endless WebKit crashes continue, and I'm down to my last idea: move as much
as possible out of the UIWebView. Rendering mustache templates in Objective-C
is the first move. As an added benefit, it cleans up the build process a bit.

This commit also introduces the concept of a "view model" to Awful. These are
model objects for a Mustache template. They're handy for compound properties
(those derived from several properties on the underlying model object), default
values, and formatters. There's one for posts and another for profiles.
Fix broken posts view actions.

Accidentally broke the post action button, following URLs, etc.
Remove empty "edited by" paragraphs in posts.

SA sends down a <p class="editedby"> regardless of whether a post is edited.
When we use JSON data, however, we don't render such a paragraph. Removing it
from the HTML makes things more consistent: the paragraph only exists when it's
nonempty.