Move zero group i18n messages to ZeroRatedMobileAccess extension

These are not Wikimedia-specific. Having these in a different
place from this extension causes UserTest::testAllRightsWithMessage
in core to fail if you have one or the other extension enabled.

See I4299d78 for corresponding change in ZeroRatedMobileAccess.

Change-Id: I49e2b2a1ae8896d8f69ea1dc22b15f9feeb17d97