R1:2d495e97014d

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…