[ticket/17316] Add template events to ucp_groups_manage

[danieltj27]

Add new template events to the UCP group management page allowing extension developers to replicate group management forms from the ACP into the UCP.

Checklist:

• Correct branch: master for new features; 3.3.x for fixes
• Tests pass
• Code follows coding guidelines: master and 3.3.x
• Commit follows commit message format

Tracker ticket:

https://tracker.phpbb.com/browse/PHPBB3-17316

[danieltj27]

I remembered to include the EVENTS.md file changes this time 😂

Also, I made sure to match the events that already exist in prosilver which use the <!-- xyz --> syntax as opposed to the {% xyz %} syntax used elsewhere. I hope that it okay!?