Fix highlighting logged-in user's quotes.

Forgot which element we applied the "mention" class to.
Fix removing empty editedby paragraphs.

They're not empty, they just have whitespace.
Don't add blank lines if adding a quote to an empty reply.

Bonus: nearby formatting.
Don't bother trying to load a thread tag if we can't parse the URL in a private message folder.
Don't crash loading a theme for a thread whose forum is unknown.
Fix crash tapping on homepage in a user's profile.

The homepage comes as a URL already, not as a string.
Fix posting threads that have secondary tags.

We weren't scraping the name of the secondary tag input. When attempting to
post, we'd try to set a value for the nil key and crash.
Replace SVProgressHUD with MRProgressOverlayView.

Looks nicer, and has a nicer API too.