Fix crash on device when built for Beta or App Store.

rdar://18315383
Formatting, formal properties, and stop rethemeing thread lists every appear.
Drop and gimme 20 private!
Tidy up thread cells.

* At least 75 points tall. Makes them look more uniform.
* Fix the cell separator: with tag visible, by setting the separatorInset properly; with tag hidden, by setting the cell content view's layoutMargins appropriately.
* Replace "text container" view with top/bottom spacer views.
* Set a minimum gap between the text and the top/bottom of the cell.
Tidy up forum cells.

* Minimum height 44pt.
* Fix separator inset.