Improve autocomplete behavior when typing ordered lists

Summary:
Ref T10163.

• If a user types an autocomplete character ("@" or "#") and then a space, deactivate immediately (probably an ordered list).
• If a user types an autocomplete character indented on a line with no other prior text, don't activate (probably an ordered list or code block).

Test Plan:
Typed:

• # , saw immediate deactivation.
• #, saw no activation in the first place.
• #x, saw activation.
• asdf #x, saw activation.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10163

Differential Revision: https://secure.phabricator.com/D15033