Show anoneditwarning when $wgMFAnonymousEditing = true

New message mobile-frontend-editor-anoneditwarning parsed
in EditorOverlay.
Don't limit to alpha.

Follow up: I8ec64e0255b93195289ec7a76a6df4ce166f6f8d

Change-Id: I33e550a4489339861029f431ad416ad948d47202
Merge "Show anoneditwarning when $wgMFAnonymousEditing = true"