Better text for <anoneditwarning> in core

The message has been tweaked to be closer to the version used for several
years with success on the English Wikipedia and on some other wikis. In
particular, the calls-to-action now have individual links rather than
expecting users to be able to magically find them themselves, and a user-
facing justification for why they should log in.

Change-Id: I687e47c23618e536a8f3010d7a6f168feb7bfa72