Allow users to create groups

[teampolyglot]

GoodReads and Twitter do this. This will allow existing close groups anywhere to start sharing learning resources from each other.

Some thoughts:

• Any user can create a new group and add other users to it (by LA username or by email?).
• Invited users must accept the request first.
• Invitations can also be created via a link, so that members don't need to be existing signed up users.
• Every group should get a public page that shows the list of users and their activities.
• Perhaps some of the users could be made admins which gives them the ability to add or remove group members.
• Groups can have name, description, image_url and website_url
• Perhaps groups can have their own ActivityPub feeds.

[teampolyglot]

Adding a boolean attribute is_public to groups table but keeping it false by default.

Basic flows for browsing, inviting members and accepting invites is now complete. For now, the creator will be the only admin, and there are no provisions to remove members or delete groups. Lets see if users like this feature at all.