[ticket/11863] Grey out unavailable activation methods when emails disabled

Further fall back to USER_ACTIVATION_DISABLE when emails are disabled
but evaluate it at runtime.

PHPBB3-11863
[ticket/11863] Display warning to user because of activation method

PHPBB3-11863
[ticket/11863] Add functional test

PHPBB3-11863
Merge pull request #2765 from Elsensee/ticket/11863

[ticket/11863] Set require_activation to none if emails are disabled