Allow selecting text when creating inline comment

Summary:
It is currently not possible to select source code covered by reticle when creating comment.
This diff hides reticle on mouseout from reply area.

Test Plan:
Hover inline comment, verify that reticle is displayed.
Reply, verify that reticle is displayed when mouseover reply, hidden otherwise.
Repeat for create.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1851