Replace get_current_user/is_valid_session with request.user

tests/test_invite.py

tests/test_mfa.py

tests/test_oauth2.py

tests/test_rolemod.py

tests/test_selfservice.py

tests/test_session.py

tests/test_signup.py

tests/test_user.py

uffd/invite/templates/invite/list.html

uffd/invite/templates/invite/use.html

uffd/invite/views.py

uffd/mail/views.py

uffd/mfa/views.py

uffd/oauth2/views.py

uffd/role/views.py

uffd/rolemod/views.py

uffd/selfservice/views.py

uffd/services/views.py

uffd/session/__init__.py

uffd/session/views.py