refactor common code, reproduce #24

- add register_user method to functional_tests.base.BaseCoffeeStatsPageTestMixin
- implement extract_link in functional_tests.base.BaseCoffeeStatsPageTestMixin
- split tests on registration page into several test methods
- add test to reproduce error described in #24
add missing load staticfiles to fix #24
finalize password reset test
redirect to profile after login if no next page is specified
fix password reset templates

- add templates/registration/password_reset_complete.html to be shown after the
  password reset
- change templates/registration/password_reset_confirm.html to use the correct
  post URL and show an error message if the reset link is not valid