[feature/oauth] Fall back to DB login if OAuth is enabled but not requested

PHPBB3-11673
[feature/oauth] Initial step in creating OAuth login support

PHPBB3-11673
[feature/oauth] Possible way of getting the login data to login_box()

PHPBB3-11673
[feature/oauth] Initial UI element added, this is non-final

At this point, all UI changes are purely for testing and should not
be considered final.

PHPBB3-11673