Saved draft overhaul and regression fixes.

Save draft overhaul:
Replies are saved in the DB, but only if needed instead of the old method that used the DB for every stage of the reply process. A popup will confirm if the user wants to use the saved draft, but inside the reply activity instead of the thread activity. If the user cancels, it'll just keep the new reply context. The draft logic is smarter, it won't try to recover a quote when you are editing or vice versa.

Removed icon-theme setting since it's completely obsolete and unused.

Improved view-posts-by-user:
-You can hit back to return to the normal thread.
-Display small inline header showing that you are browsing by a single user, and instructing that you can hit back to return.

Regression fixes:
-Crash when displaying alerts started from secondary thread.
-Fix view-posts-per-user crash and having the wrong last page number.
-Fix potential crash when creating a second alert before a previous alert finished displaying. New alerts automatically close the previous one, so they won't overlap.

Misc:
-Progress alert for mark-last-read.