Add Campaigns & CoreEvents extensions

Campaigns is only enabled if EventLogging is present, and only on labs and test
wikis for now. CoreEvents is enabled anywhere EventLogging is enabled.

I also added a comment denoting the end of general-purpose extensions, as
people seem to add them in chronological order.

Change-Id: I7c45ebd1c732ccf580e283e47a8bf8cc7bdfe186