A few new tests

[dotsbb]

Few functional tests for modern flows (sign in & sign up):

• sign up with the same data for second time
• login with wrong credentials
• login to [de]activated user's account
• more than one user with this e-mail
• trying to sign up using the same credentials twice
• check if templatetags works fine

[idlesign]

Merged. Thank you.

BTW, 1.4 doesn't have get_user_model, so do not forget to run tox ;)