Handle Forums links to a single user's posts in a thread and rearrange URL handling.
Write our own split view controller.

Fixes #206.
Fix selecting ticked setting in settings choice table.

This is the table that appears when you tap a setting that has multiple
choices, like the "Keep Sidebar Open" setting.
Fix showing single user's posts.

Without filtering by user ID too, we were getting all posts we'd ever viewed in
the context of a single user's posts.
Push a single user's posts on to the navigation stack.

Very simple solution to the "how to exit the filter" question from @diabolik900
and @ridilla.