Add new setting to make Group read only

[Venutius]

There was an ask that came up in the forums that I thought might be considered for a feature addition to core.

The ask was to make a group read only so that only Group admin could post to it and new members could not join.

I think that as sites get older then this requirement would come up more often and it seemed to me that this could be a new setting in the Group Privacy Options - Closed Group - read only, with an additional option that determined whether a group could be joined by new members or not.

[DJPaul]

Implementation details aside, I like this idea. Thanks for suggesting it, @Venutius!

[dcavins]

This sounds like another permission set of what I'm trying to accomplish in #6677. I'm going to close this ticket in favor of that ticket, but mention this ticket there.