Monthly active users -> Active users last 30 days

Clarify what `Monthly active users` means by changing to `Active users last 30 days`.

app/views/admin/dashboard/index.html.haml

@@ -94,7 +94,7 @@
         %span.light.pull-right
           = Milestone.count
       %p
-        Monthly active users
+        Active users last 30 days
         %span.light.pull-right
           = User.where("current_sign_in_at > ?", 30.days.ago).count
     .col-md-4