Typeahead - filter typeaheads that the viewer can't see in typeahead debug tool

Summary:
Fixes T7255.

Note however that some datasources - notably user or project - don't implement the class thing in a clean way since multiple classes apply. For now, we just show these datasources to the user.

Also, I guess this could be done more efficiently by querying for all the applications at once via an application query? LMK if you want me to make that change.

Test Plan: loaded /typeahead/class/ and played with it a bit with no issues

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7255

Differential Revision: https://secure.phabricator.com/D12307