Fixed #13087 -- Modified m2m signals to provide greater flexibility over exactly when notifications are delivered.

This is a BACKWARDS INCOMPATIBLE CHANGE for anyone using the signal names introduced in r12223.

 * If you were listening to "add", you should now listen to "post_add".
 * If you were listening to "remove", you should now listen to "post_remove".
 * If you were listening to "clear", you should now listen to "pre_clear".

You may also want to examine your code to see whether the "pre_add", "pre_remove" or "post_clear" would be better suited to your application.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12888 bcc190cf-cafb-0310-a4f2-bffc1f526a37