Restore Signupstart and Signupend messages for account creation

This was removed during the redesign of the login process in
I50f25583, but I found that 5-10% of non-wmf wiki sites used this
message to display notices, previously.  We should keep this around or
at the very least provide advance notice for people.

Bug: 56455
Change-Id: I79423c1a05f0359c902d1940c3ffcd5e509dcf97
(cherry picked from commit 8cfedc494ab6dcdfd0c0ac8202b1685b2ab9581f)