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