Issue #1934568 by David_Rothstein, pwolanin: Allow sites using the 'image_allow_insecure_derivatives' variable to have partial protection from the security issues fixed in Drupal 7.20.
Prepare Drupal 7.21 release.
Merge tag 'tags/7.21' into drupal-7.21

7.21 release
Merge branch 'drupal-7.21'
Update to Drupal 7.21 - maintenance release focusing on users impacted by the image derivative changes in 7.20. For more information see:

http://drupal.org/drupal-7.21
Merge branch 'master' of git://github.com/pantheon-systems/drops-7