[1.2.X] Fixed #14477 -- Corrects several instances of "add" and "remove"...

[1.2.X] Fixed #14477 -- Corrects several instances of "add" and "remove" m2m_changed signal actions not having been updated to "pre_add", "post_add", etc. Thanks to slink and andrews for reporting several instances of it, and andrews for the draft patch.

Backport of [14304] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14305 bcc190cf-cafb-0310-a4f2-bffc1f526a37