Use bugs email from settings, not hardcoded value

lms/templates/registration/activation_invalid.html

@@ -12,7 +12,7 @@
     <p>Something went wrong. Check to make sure the URL you went to was
       correct -- e-mail programs will sometimes split it into two
       lines. If you still have issues, e-mail us to let us know what happened
-      at <a href="mailto:bugs@edx.org">bugs@edx.org</a>.</p>
+      at <a href="mailto:${settings.BUGS_EMAIL}">${settings.BUGS_EMAIL}</a>.</p>
 
     <p>Or you can go back to the <a href="/">home page</a>.</p>
   </section>