Store inline comment offset information and show it when highlighting comments

Summary:
Ref T13513. When a user selects a text range and uses "New Inline Comment" to create a comment directly from a range, store the offset information alongside the comment.

When hovering the comment, highlight the original range.

Test Plan:

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21250