Warn and prevent 1-up/2-up switch in Differential if the user is editing an inline

Summary:
See PHI180. Currently, if you begin creating or editing an inline and then swap display modes (for example, with "View Unified"), your edit is lost.

Persisting the editor state is complicated and this is very rare, so just prevent the action and warn the user instead.

Also make the warning persist for a little longer since a few of the messages, including this one, take a couple seconds to read now.

Test Plan:

• Edited a comment, tried to swap display modes, got a warning.
• Swapped display modes normally with no comment being edited.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18774