Keep loading screen up until next page of posts has rendered.

Previously we'd get a flash of the last page before the new page showed up.
Commentary.
Properly detect URLs with the alternate host as Forums URLs.
Fix ordering of cancel button in link long-press menu.
Fix crash when dismissing preview screen before network operation finishes.
Delete pointless argument.
Temporarily add log message to crash reports regarding "tried to present modally an active controller" exception.
Add logging to crash reports for Crashlytics issue #6418.

-[AwfulForumsClient listPostsInThread:...] getting a nil threadID when called
with a thread passed from the AwfulComposeTextViewControllerDelegate
implementation in AwfulForumThreadTableViewController.
Stop rendering cached irrelevant posts when loading the next unread page of a thread.
Rap Sheet and Leper's Colony dark theme.

Plus some cleanup while I was in town.
Fix truncated Rap Sheet/Leper's Colony reasons.

Reading the documentation can prove enlightening!
Set a UIKit tint color for dark mode.
Set theme's tintColor on window when launched.
Setting to enable forum-specific themes outside of the intended forum.

Toggleable per-theme because two lines of theme choices in a posts screen is
ugly.
Reorder a theme key.
Fix calculating height of footer views in settings screen.
Remove now-unused AwfulSettingsChoiceViewController.
Fix spurious top margin above alternate server table.
Don't bother coloring the footer on the ubiquitous theme picker, looks dumb.
!!