Delete unused class AwfulScreenCoverView.
Delete unused #import.
Semi-modal view controller.

Presents itself next to some part of another view controller, dimming the rest
of the presenting view controller's contents.
Reimplement icony action sheet as semi-modal view controller.

* AwfulPostsView can say where a post's header, footer, or action button is.
* Icony action sheet makes no use of auto layout anymore. Wasn't worth the pain.
Remove circles around action icons in sheets.

Bonus: directly exposed the title label and icon image view on the cell to
simplify implementation.

Bonus 2: Action items always return template images for icons.