Fix notifications in thread list.

Ensure they come in on the main thread, and that they're unregistered in
-dealloc.
Formatting.
Don't try to update thread tags on missing cells.

This can happen if e.g. we look for a tag for a page 2 thread, then refresh the
thread list, so the page 2 thread's cell isn't shown anymore. I think this
results in a crash.
More external browsers.