Add Post#title; don't extract title from HTML *all* the time... :)
Include post titles in JSON.
Only render Markdown to HTML for posts by hosted users; only run the existing HTML through the sanitizer for everyone else.
Add h-entry microformat markup to posts.
Also make the post author a h-card.
Track URLs referencing posts.

* Adds a new Post#referenced_by array
* Incoming pings store their source URL in this array
* It will serve as the base for all reference/reply/conversation tracking
Include post title on post pages (but invisible. For p-name only.)
Include #title in attributes PostFetcher will copy from remote post.
Include p-category in hashtag links.
Fix dt-published to include ISO8601 timestamp.
Apply u-photo microformat to post author avatar.
Apparently, u-photo can only be applied to IMG tags. :/
Fix mf2 by moving avatar into h-card element.
Add `avatar_image_tag` helper.
Improve/fix h-card p-author microformat.
Add an API that is actually useful.

See https://github.com/hmans/panties
Fix hashtag links.
Add "load more posts" to tag pages.
Remove Post#sha and Post#previous_shas.

Not used anywhere, and will eventually be replaced with a much smarter key-based signing mechanism.
Tie primary navigation entries to CanCanCan abilities