Fix page settings sheet: height, theme buttons, and title placement.

* There should not have been a margin above the title label.
* The height was being calculated incorrectly, causing the theme picker to have
  a very short frame. It still drew the buttons outside its frame, but the
  touch targets were very small.
Themes.plist keys for new action sheets.
Pin jump to page sheet buttons to center of their row.

Their layout was ambiguous with no vertical positioning.
Set default theme for forums with their own themes.

* Defaults go into the hidden section of Settings.plist.
* Attempting to set a forum's theme to its default will instead clear the saved
  value from the user's preferences.
* No need to pick a default YOSPOS theme when migrating old settings.
* Bonus: add a description to the BYOB theme.
* Bonus: some formatting in AwfulSettings.m.
* Bonus: DRY up deriving form-specific settings keys.
Action sheet color tweaks

Iiiiiiiii don’t know about those.