Allow prioritizing QUIT over other commands

Add a forceImmediate flag to issueQuit, allowing the QUIT command to
jump the command queue.

A future update will prioritize QUIT when core is shutting down so
other messages won't block showing the user's quit message.

Don't prioritize QUIT normally in case the user sends some messages,
thinks they've all been delivered, then sends a QUIT.

(Eventually, Quassel should display queued messages differently from
messages that have been sent)

Closes GH-201.

(cherry picked from commit 3966090a1e7093c417560f7ee13ab310215d9ccd)