Fix an exception that was thrown when trying to autocomplete saved
search labels (e.g. `search:all`) as an anonymous user. This was a
pre-existing bug.
Reworks tag autocomplete to work the same way for all users. Previously
autocomplete for Builders worked differently than autocomplete for
regular users.
This is how it works now:
* If the search starts with a slash (/), then do a tag abbreviation
match. For example, `/evth` matches eyebrows_visible_through_hair.
* Otherwise if the search contains a wildcard (*), then just do a simple
wildcard search.
* Otherwise do a tag prefix match against tags and aliases. For example,
`black` matches all tags or aliases beginning with `black`.
* If the tag prefix match returns no results, then do a autocorrect match.
The differences for regular users:
* You can abbreviate tags with a slash (/).
The differences for Builders:
* Now tag abbreviations have to start with a slash (/).
* Autocorrect isn't performed unless a regular search returns no results.
* Results are always sorted by tag count. Before different types of
results (regular tag matches, alias matches, abbreviation matches,
and autocorrect matches) were all mixed together based on a tag
weighting scheme.
This refactors the autocomplete Javascript to use a single dedicated
/autocomplete.json endpoint instead of a bunch of separate endpoints.
This simplifies the autocomplete Javascript by making it so that instead
of calling a different endpoint for each type of query (for users, wiki
pages, pools, artists, etc), then having to parse the results of each
call to get the data we need, we can call a single endpoint that returns
exactly what we need.
This also means we don't have to parse searches clientside in order to
autocomplete metatags. Instead we can just pass the search term to the
server and let it parse the search, which is easy to do serverside.
Finally, this makes autocomplete easier to test, and it makes it easier
to add more sophisticated autocomplete behavior, since most of the logic
lives serverside.
