As a super admin, I want to know the special email address for each group, so that I can send to it

[camjackson]

Background:

Our first cut of the "send an email to a group" works by having a super admin send their email to a magical email address representing a specific group, and we then forward that email to all the relevant members.

There are other cards to implement all of the actual email handling, but at the moment there's no way for the super admin to know what the magical email address is. This card adds that information to the UI.

Acceptance criteria:

• There should be a mailto: link for the currently selected group
• The user should be the group's ID*
• The domain should be the domain of the Rabble Rouser instance**
• The link should only show for super admins, not branch admins
• The link should only show for 'real' groups, not ones like "Unassigned" and "All members"
• The super admin needs to know that:
• They can send the email using whatever mail client they like
• The subject and body that they send will be forwarded as-is
• They must send the email from the address that they use to log in to Rabble Rouser

• This is available in the Redux store on the frontend
  ** This isn't explicitly anywhere in the frontend code. Probably easiest to use window.location.host