Fix up cross-version syncing and registration with 0.10.3 downgrade

@aronasorman These are really necessary for 0.10.3; committing them here, but will be patched to 0.10.3 (and should be tested there)

Issue 1:
* 0.10.3 central / 0.10.3 distributed server weren't syncing properly, because properties with `minversion='0.11.1' weren't being synced, but were included in the schema.
* Fix: reset the minversion to 0.10.3

Issue 2:
* After registration with the central server, if a central server login redirect was necessary, the user is redirected... back to the central server login page, instead of back to the distributed server registration page.
* Fix: propagate "prev" / referer through the login redirect, so it can be properly used by pages downstream from the `login_required` decorator

Testing:
* Set up the 0.10.3 servers, apply these commits
* Register the test distributed with test central server, without being logged into the central server before starting the process (you'll be prompted for login during the process), and make sure you're redirected after successful registration to the distributed server registration page.
* On the distribute dserver, run `generaterealdata`, then `syncmodels`; verify there are no sync errors.

@aronasorman, this is all you.  After this is done, I think we're ready to release.