Remove "^" (Prev) and "V" (Next) actions on Differential inline comments

Summary:
Ref T12616. Fixes T12715. I suspect these are very rarely used. (I think you tried to get rid of them before but I pushed back since we couldn't really offer great alternatives at the time?)

Now that the code is in a better place:

• Click an inline's header (just the colored part) to select it with the keyboard selection cursor.
• Click again to deselect it.
• You can use "n" and "p" to jump to comments, so "click + n" is the same as the old "V" action.
• This also makes it easier to swap between keyboard and mouse workflows, since you can jump into things with the keyboard at any inline.

Also, make "Reply" render more consistently.

Test Plan:

• Did all that stuff, things seemed to work OK.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12715, T12616

Differential Revision: https://secure.phabricator.com/D17908