Remove anchors from AnnouncementLink and Entry

These models were used in some templates to control when prominent
announcements appeared.  We had anchors so that announcements could be
posted to individual programs and classes, but this feature was never
used (to my knowledge).  So, I removed the anchor and made the migration
remove any announcements that weren't "public" (i.e. anchored to Q/Web).
A couple of views and templates have been modified to exclude other uses
of announcements.
Convert inline_qsd_block to inline_program_qsd_block in two places
Add Django Group to admin site
Fix DataTree reference in ESPUser.addAvailabletime
Remove esp.cal.models.Series model

It had a DataTree reference, and we weren't using it anywhere.  (Of
course, we may want to bring back the concept of a 'series of events' at
some point, but we should do so with a new model.)
Make ForeignKeys to the DataTree allow null values

As part of the transition away from the DataTree, we are going to be
saving model instances without DataTree anchors.  In order for this to
work, all of our anchor fields must allow NULL values.

This raises an interesting question, which is whether these migrations
should be reversible.  In order for them to be reversible we'd have to
write some fairly interesting data migrations.  So far we have made them
irreversible.
Fix UserAvailability migration: get_or_create to make new Groups
Remove unused, UserBit-dependent ClassSubject.subscribe()
Update a few lingering uses of program anchor to use name/instance properties
Order class sections by ID instead of anchor name
No longer post a miniblog Entry when approving a class
Update some code associated with locating QSD pages for a class
Change the way Media objects are associated with programs/classes

Per-class and per-program document uploads were associated with the
class/program through matching DataTree anchors.  This has to go, so I
added a generic field on Media that is restricted to point to only
ClassSubject or Program.  The class documents features of teacher
registration and the AdminMaterials module have been updated to reflect
the change.
Remove a few unneeded 'anchor' mentions from Resource
Remove more anchor references from Class* models file
Remove non-useful Program.teacherSubscribe() function
Add program filtering back into Resource grouping functions

This will make it easier for existing code.

This is a partial revert of 46aa7e1.
Note that this code is all going away anyway.
Remove Datatree references from Program models code
Remove unused template tag (generate_event_list) for esp.cal app