Refactor groupJoin

[NikkoJT]

The way groupJoin works right now is pretty bad - performance-inefficient and cluttering the action menu.

I'd like to look at alternative solutions. One option could be moving it into a briefing tab which shows a list of links to join groups on the same side that have players in, which occasionally updates. (The list could be range-limited to prevent abuse in missions with deliberate separation)

• 43 Briefing button for overriding group leadership. #412 could be adjusted to live in the same "group management" tab

[costno]

I like the concept but I think the addAction is super easy to communicate to new players in real time: "hey if you look at me you can join my group with an action".