(mingle #405) Add dynamic head msg on login/acct create form

* Preserves 'sign up', 'sign in' overlay headers on login/acct create form
* Sets head msg to 'You must be logged in to watch pages.' when login/acct
create form reached from tapping watchlist start
* Sets head msg to 'You must be logged in to see your uploads.' when
login/acct create form reached from Special:DonateImage

Change-Id: I07b1be58ae227d973d4981819da6a8dd38e77142
Merge "(mingle #405) Add dynamic head msg on login/acct create form"