Fix crash on settings tab when username looks numeric.

JSON comes from the forums with numeric-looking usernames encoded as numbers.
Move plain bar button item out of Posts.
"Post new thread" navbar button.
Thread compose view.
Enable all text boxes in PM compose view, not just the big text view.
Fetch available thread tags when posting new thread.
Formatting.
Parse and POST new thread form.
Thread compose controller delegate includes a thread ID.