[ticket/12831] Optn to prevent setting HTTP headers

https://tracker.phpbb.com/browse/PHPBB3-12831

PHPBB3-12831
[ticket/12809] Fix select button and dropdown list items in RTL.

PHPBB3-12809
[ticket/12809] Remove obsolete rules and selector.

PHPBB3-12809
[ticket/12809] Restore text-align for the pagination.

PHPBB3-12809
[ticket/12822] Remove "None" option from avatar select boxes

This option is just used for deleting avatars for which we already have
a checkbox.

PHPBB3-12822
[ticket/12822] Add method for deleting avatars and use confirm_box

The method will take care of the actual deletion process in the avatar manager.
Additionally, we'll be using a confirm box while deleting the avatar to prevent
users from accidentally deleting their avatars.

PHPBB3-12822
[ticket/12822] Fix test runs of avatar manager_test

PHPBB3-12822
[ticket/12671] Possibility to use NOT LIKE expression

PHPBB3-12671
[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/12794] Properly validate google+ field against valid character set

PHPBB3-12794
[ticket/12671] Possibility to use NOT LIKE expression

PHPBB3-12671
[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/12671] Possibility to use NOT LIKE expression

PHPBB3-12671
[ticket/12671] Possibility to use NOT LIKE expression

PHPBB3-12671
[ticket/12948] Remove no longer needed opcache workaround. Travis has 5.5.15.

PHPBB3-12948
[ticket/12941] Add sami.conf.php for Travis CI.

PHPBB3-12941
[ticket/12941] Check for Sami parse errors on Travis CI.

PHPBB3-12941
[ticket/12938] Remove the 'Download all attachments' feature.

PHPBB3-12938