Fix category prefix metatags not working in autocomplete. Now typing
e.g. `copy:t` will show tags starting with 't' in autocomplete.
Also fix it so that tags beginning with a '(' work in autocomplete.
Typing e.g. `-(tou` will show `touhou` in autocomplete.
This also fixes it so that when you type a negated tag in autocomplete,
e.g. `-touhou`, it sends `touhou` in the autocomplete API call, rather
than `-touhou`. This makes caching more effective since negated tags
will be cached the same as non-negated tags.
031ab1e833