[ticket/11930] Use \phpbb\path_helper for avatar URLs

This will ensure that avatars still properly display on extension pages
supplied via app.php.

PHPBB3-11930
[ticket/11930] Fix tests after adding phpbb\path_helper to avatar drivers

PHPBB3-11930
[ticket/11930] Move path_helper in front of optional cache argument

Optional method arguments should be after any mandatory ones.

PHPBB3-11930
[ticket/11930] Modify order of properties to fit constructor method

PHPBB3-11930
[ticket/11930] Update docblock of avatar driver constructor

PHPBB3-11930
Merge remote-tracking branch 'marc1706/ticket/11930' into develop

* marc1706/ticket/11930:
  [ticket/11930] Update docblock of avatar driver constructor
  [ticket/11930] Modify order of properties to fit constructor method
  [ticket/11930] Move path_helper in front of optional cache argument
  [ticket/11930] Fix tests after adding phpbb\path_helper to avatar drivers
  [ticket/11930] Use \phpbb\path_helper for avatar URLs