+ Tag.slug index, oops!

Turns out `unique_together` only creates a multi-value index, not an
index on each attribute (like I'd incorrectly assumed)
Tag autocomplete should use slug (index)
Replace inefficient, slow link count query.

The query before wasn't actually doing anything different from a simple
count, and was taking ~14 seconds to run in some cases!  Replaced with
a simple, faster count.