Provide an instance-wide config equivalent to prevent external invitations from a group

Release notes

The settings Prevent members from sending invitations to groups outside of Group and its subgroups can now be applied globally at an instance level, instead of being applied per-group.

Problem to solve

On self-managed instances with tight controls on membership controls, the setting Prevent members from sending invitations to groups outside of Group and its subgroups under a group is very useful.

However, being a per-group settings its not easy to keep it consistently applied to all created groups.

It'd help if this and similar membership control settings are made instance-level for self-managed instance users.

Proposal

Offer a version of Prevent members from sending invitations to groups outside of Group and its subgroups as an instance level settings that overrides or influences the per-group settings for all existing and new groups.

Intended users

• Sidney (Systems Administrator)

Metrics

TBD

Related to #348390 (closed).