Improve select-to-comment behavior in Firefox and on unified diffs

Summary:
Ref T13513.

• Firefox represents multiple selected rows as a discontinuous range. Accommodate this.
• Unified diffs don't have a "copy" marker. Do something sort-of-reasonable for them.

Test Plan:

• Selected multiple lines of content in Firefox, got an option to add a comment.
• Selected content in unified mode, got an option to add a comment.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21249