Added placeholder release notes for the 1.2.1 and 1.2.3 releases.

Both of these releases were to fix bugs in the previous release and, as
such contained nothing new of significance. However, to complete the set
of release note files in case anybody worries about the gaps, we now
have documents explaining precisely that.

Fixed #15330.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16695 bcc190cf-cafb-0310-a4f2-bffc1f526a37