[ticket/12511] Add missing titles for profile fields in members group view

For memberlist group view in cases the group is presented with the group leader
and group members, for members profile fields titles are missing: Posts,
Joined, Last active and custom profile fields titles.

PHPBB3-12511
Merge pull request #2423 from rxu/ticket/12511

[ticket/12511] Add missing titles for profile fields in members group view

* rxu/ticket/12511:
  [ticket/12511] Add missing titles for profile fields in members group view