Move code that runs after user save from save_new_user to create method

[AntonKhorev]

Currently save_new_user does other things like redirects after the actual save. That obscures which pages are reachable from actions that call save_new_user, see #5431 (comment).

Also save_new_user accesses referer in two different ways: as a function argument and as params[:referer].

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Added test for clicking Sign Up, filling out email-login-password, clicking Sign Up, opening confirmation link from email.

[tomhughes]

I mean it's not really clear why save_new_user exists when it's only called once but I guess this fine.

[AntonKhorev]

save_new_user was called from two different places when if was first introduced. The other was I think save after terms page.