Give selected inline comments are more obvious selected state

Summary:
Ref T13513. Give selected inlines a selection state and visual cues which are similar to the changeset selection state.

Also fix a couple of minor issues with select interactions and offset comments.

Test Plan: Selected inlines, saw obvious visual cues.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21256