users: move emails to separate table.

* Move emails from users table to email_addresses table.
* Validate that addresses are formatted correctly and are unique across
  users. Existing invalid emails are grandfathered in.
* Add is_verified flag (the address has been confirmed by the user).
* Add is_deliverable flag (an undeliverable address is an address that bounces).
* Normalize addresses to prevent registering multiple accounts with the
  same email address (using tricks like Gmail's plus addressing).

app/views/users/edit.html.erb

@@ -25,7 +25,13 @@
         <div class="input">
           <label>Email</label>
           <p>
-            <%= CurrentUser.user.email.presence || "<em>blank</em>".html_safe %> – <%= link_to "Change your email", new_maintenance_user_email_change_path %>
+            <% if @user.email_address.present? %>
+              <%= @user.email_address.address %>
+            <% else %>
+              <em>blank</em>
+            <% end %>
+
+            - <%= link_to "Change your email", new_maintenance_user_email_change_path %>
           </p>
         </div>