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 can wait for a better use case for letting you autocomplete milestones, I think.

Also, don't try to cycle hashtags when renaming milestones. This was a little inconsistent before.

Test Plan:

• Autocompleted normal projects.
• Autocompleted milestones with explicit hashtags.
• No autocomplete entry for milestones with no special hashtags.
• Used normal typeahead to get tag-less milestones to make sure I didn't break anything.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10285

Differential Revision: https://secure.phabricator.com/D15261