Add centralauth-welcomecreation-msg

People want to be able to specify a message to be displayed after
account creation, so add centralauth-welcomecreation-msg to allow them
to do that.

We also allow the CentralAuthPostLoginRedirect hook to override this
message.

Bug: 52373
Change-Id: I69d854cb2262fb6670cb8be40542937745c6e8c4