Removes unused code.

The attendance app has no views, isn't in our INSTALLED_APPS, and the .pyc files didn't even
exist so it was never imported.  Trying to import the models file errors due to
a broken import.  Seems like it's time for it to go into retirement.

esp.utils.scheduling appears to be the old HSSP lottery, which we no longer use.  esp.utils.sanity_check_schedules appears to be some old scheduling checks not touched since 2010; now we have an actual scheduling checks page that does all this.  esp.utils.classchange appears to be some old class changes scripts.  @jmoldow, I assume this is subsumed by the newer class changes stuff?  A number of other esp.utils.* files were never actually used, or no longer actually used after removing stuff above.

esp.users.tests-defunct is not even a valid module name and hasn't really been touched since 2008.  esp.web.util.structures and some functions in esp.tests.util were only used in it.

esp.lib.TOC doesn't appear to be used anywhere and hasn't been touched in 5+ years.

Several middlewares were unused.

esp.shortterm should be self-explanatory.  The venerable crazy_excel_thing was preserved in useful_scripts under a more useful name.

A bunch of stuff in esp.db was unused, and probably died with the datatree.

esp.cal.Lists_EventType was unused.  esp.cal.show_calendar appears to have had some views, but they have no URLs so must be unused.

Also removed a bunch of unused imports (which I found with pyflakes).  In a couple of files with a lot of late-bound imports I moved them to the top when possible, because often things were getting imported multiple times.