Customize the Devise `password_change` emails

app/views/devise/mailer/password_change.html.haml

+.center
+  #content
+    %h2 Hello, #{@resource.name}!
+    %p
+      The password for your GitLab account on
+      #{link_to(Gitlab.config.gitlab.url, Gitlab.config.gitlab.url)}
+      has successfully been changed.
+    %p
+      If you did not initiate this change, please contact your administrator
+      immediately.

app/views/devise/mailer/password_change.text.erb

+Hello, <%= @resource.name %>!
+
+The password for your GitLab account on <%= Gitlab.config.gitlab.url %>
+has successfully been changed.
+
+If you did not initiate this change, please contact your administrator
+immediately.