[ticket/12596] Add template event acp_group_options_before/after

Add template events to the group settings page to allow extensions to
add
additional options for groups.

PHPBB3-12596
Merge pull request #2502 from tmbackoff/ticket/12596

[ticket/12596] Add template event acp_group_options_before/after
Merge branch 'develop-ascraeus' into develop

# By Tabitha Backoff
# Via Dhruv Goel (1) and Tabitha Backoff (1)
* develop-ascraeus:
  [ticket/12596] Add template event acp_group_options_before/after