R1:df272dfa266b

Allow autocomplete to activate after ">", ":" or "!"

Summary: Ref T10163. This allows activation in blockquotes or after `NOTE:`, for consistency.

Test Plan: Typed `>@dog`, etc., got autocomplete.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.com/D15035