(bug 43909) (mingle 377) Move watchlist data prep for login form to private method

This is in preparation for being used for account creation as well
as login.

Change-Id: Ie01bc7f7ff97cb67d7c7e5e0991e0c7a8aafe7d3
Merge "(bug 43909) (mingle 377) Move watchlist data prep for login form to private method"