[ticket/11444] Moving the in-board notifications to it's own method class 

Currently the in-board method for the notifications is hardcoded and
cannot be disabled. This method should be in his own class extending
`phpbb\notification\method\method_interface`.

It also add the possibility, for each method, to be enabled by default (ie:
no entry in the DB => notification enabled) like for the in_board one.

https://tracker.phpbb.com/browse/PHPBB3-11444
https://tracker.phpbb.com/browse/PHPBB3-11967

PHPBB3-11444