Improve autocomplete behavior in lists and with noncompleting results

Summary:
Ref T10163. Currently, we don't activate on indented lines, but were too aggressive about this, and would not activate on lines like - Hey, @user..., where we should.

Instead, don't activate on indented lines if there's only an indent (i.e., # probably means enumerated list).

Also, if results don't have autocompletes (rare but possible with projects missing slugs), improve behavior.

Test Plan:

• Typed #a, got no autocomplete.
• Missing slug thing is a pain to test locallly, #1 z z z z reproduces in production. I'll just verify it there.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.com/D15040