UI to approve users who have signed up

[ddey2]

#973 provides a way to flag users in keycloak for being enabled/disabled. Need to work on UI controls.

[max-zilla]

Are there things we want to do differently from Clowder1 for this?

• automatic approval flag
• option to send digests? link to approve inside the email?

[max-zilla]

@tcnichol suggests inviting someone to join Clowder who will be automatically approved.
@robkooper suggests a slack-style URL where people can sign up and get auto-approved.