Configure visibility of group details (description, members, room-list)

[lampholder]

Ultimately we want to support more fine-grained controls for which rooms/people/etc. can be seen by members/non-members/etc. of a given group.

As a high-value quick win, however, we want to implement a flag on groups that dictates whether a user who is not a member of that group can see any of the group details.

I think we should avoid calling this feature 'public' or 'private' groups because:

• The existence of a group-with-details-hidden-from-non-members will still be public - even if we don't list it in a directory, users outside of the group can discover its name by trying to register it
• A 'public' group sounds like it pertains to ability to join rather than visibility of details

I think we want to:

1. add an option to group settings:
   • ☑️ Show group details (description, members, room-list) to non-members
   • This feature should default to 'on'
2. show non-members (who will still find the group through clicking on flair) a notification:
   • The details of this group cannot be seen by non-members., or

[turt2live]

Probably not part of the quick-win goal, but it would be nice to further supply a different long/short description for non-members. For joining instructions, or to show exclusive content to members of the group.

[lukebarnard1]

• Add is_public to groups on synapse - Add is_public to groups table to allow for private groups matrix-org/synapse#2582
• Add setter for is_public on synapse
• Add checkbox to group settings page

[lampholder]

We've decided that we can impl #5480 for the first cut of groups, meaning this isn't a release blocker :)