Hide inlines one-by-one, instead of in a big group

Summary:
Ref T12616. Fixes T12153. Currently, when you hide inlines, they hide completely and turn into a little bubble on the previous line.

Instead, collapse them to a single line one-by-one. Narrowly, this fixes T12153.

In the future, I plan to make these changes so this feature makes more sense:

• Introduce global "hide everything" states (T8909) so you can completely hide stuff if you want, and this represents more of a halfway state between "nuke it" and "view it".
• Make the actual rendering better, so it says "epriestley: blah blah..." instead of just "..." -- and looks less dumb.

The real goal here is to introduce DiffInline and continue moving stuff from the tangled jungle of a million top-level behaviors to sensible smooth statefulness.

Test Plan:

• Hid and revealed inlines in unified and two-up modes.
• These look pretty junk for now:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12616, T12153

Differential Revision: https://secure.phabricator.com/D17861