[ticket/12245] Fix id attributes in acp_prune_users.html

The following labels had invalid id references:
1. joined (renamed to link to joined_after)
2. group_id (create teh id attribute on select box)

PHPBB3-12245
Merge pull request #2083 from hjpotter92/ticket/12245

[ticket/12245] Fix id attributes in acp_prune_users.html

* hjpotter92/ticket/12245:
  [ticket/12245] Fix id attributes in acp_prune_users.html