Install rails_admin and devise. Start setting it up.

app/views/devise/mailer/unlock_instructions.html.erb

@@ -0,0 +1,7 @@
+<p>Hello <%= @resource.email %>!</p>
+
+<p>Your account has been locked due to an excessive number of unsuccessful sign in attempts.</p>
+
+<p>Click the link below to unlock your account:</p>
+
+<p><%= link_to 'Unlock my account', unlock_url(@resource, unlock_token: @token) %></p>