Campaign: Replace the Drain and Oilrig levels with Opium and Bromine

Depends on !129 (merged) and !135 (merged)
Also simplifies the campaign name (drop the Xonotic version number and campaign author, they're now credited in the game credits instead).

Implements #203 (closed)

Removing Drain and Oilrig was accepted in polls 371 and 384 due to performance issues and fps drops reasons, but in that case will stay hidden.

Adding Bromine and Opium was accepted in polls 382 and 383.