[ticket/12742] Add method for getting authenticated recipients

PHPBB3-12742
[ticket/12742] Add check for empty $users and add method to fitting methods

The method might now return an empty array in case of an empty users array,
too. The notification types then check if the returned array is empty and
return this if it is empty and if the notification types would otherwise
carry out other operations afterwards.

PHPBB3-12742
[ticket/12742] Replace authenticated with authorised

PHPBB3-12742
[ticket/12742] Remove unneeded sort of users

It doesn't matter which user will get the notification first so we shouldn't
sort the users array.

PHPBB3-12742
[ticket/12742] Add sort options for notification types requiring sorting

PHPBB3-12742
[ticket/12742] Add sorting back to bookmark notification list of users

PHPBB3-12742
Merge pull request #2840 from marc1706/ticket/12742

[ticket/12742] Put duplicated code into one method in notifications base