Enable admins to activate users

Add an "Activate" button next to users on the /admin/users page so that
admins can activate user accounts.

Some refactoring to avoid code duplication between activating by users
clicking on an activation link and admins clicking the new activate button:
move the code that actually activates the user onto the User model where
both views can call it.

h/static/styles/admin.scss

@@ -10,3 +10,7 @@ body {
   table-layout: auto;
   width: auto;
 }
+
+.users-activate-form {
+  display: inline;
+}