Allow users to create inline comments by directly selecting text directly

Summary: Ref T13513. Support direct text selection for inlines. This is currently just an alternate way to get to the same place as using line numbers, but can preserve offset/range information in the future.

Test Plan:

• Selected some text, hit "c", clicked "New Inline Comment", got sensible comments on both sides of a diff in Safari, Chrome, and (with limitations) Firefox.
• Caveats: no unified support, doesn't work across lines in Firefox.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21248