Add Admin UI for emailing users

[kirahowe]

Create an admin UI to send mass emails to selected groups of users. The groups should be:

• every email we have
• just people who have submitted a message to us
• just "visitors" (people who gave us their email address in the homepage email box)
• It should also be possible to select a group of any number of recipients manually (with a list of all the emails and checkboxes).
• Once the recipients are selected, admin should be able to write a message and send it.
• Store sent messages

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Notes on implementation:

• after saving either a Visitor or Message find_or_create an EmailRecipient with the email address we just got