R1:68d05934a796

Don't show un-completeable results in people/project autocomplete

Summary:
Fixes T10285.

- If a result (like a milestone) has no primary hashtag, try to fill in a secondary hashtag.
- If we can't find any hashtag, don't return the result.

This produces these behaviors:

- By default, you can't autocomplete milestones.
- If you give one a hashtag, you can.

We might want to give milestones "special" hashtags eventually (like `#xyz/33`) but this fixes the confusing/broken behavior in the UI and we…