Allow comment panel to be stuck/unstuck to the bottom of the display

Summary:
See T303. Enable comment panel haunting.

I hid the preview for the sticky panel, which I think is reasonable?

Test Plan:
https://secure.phabricator.com/file/view/PHID-FILE-64713fa8a7c2a22e5b93/
Reviewed By: broofa
Reviewers: broofa, jungejason, aran, tomo, tuomaspelkonen
CC: aran, broofa
Differential Revision: 615