Remove unused dblog code.

Looks like this app was designed to log errors in the database.  The last time
one was logged on esp.mit.edu was in 2010, so I don't think we're actually
using this functionality; let's kill it!

It was called in a few places:
* users.getTaughtClassesFromProgram attempted to log, but not raise, an error.
  This is really bad anyway so it now raises ESPError.
* the ESPError middleware attempted to save the error to the database after
  sending it to serverlog, but it was wrapped in a try-except block, and
  doesn't seem to have been happening.
* Some other navbar view imported, but did not use it.  That entire view seems
  to be unused, so I deleted it too.

I think in the long term we should probably improve our logging setup, possibly
involving a new-and-improved way of logging errors to the DB, but I don't think
we will actually want to build on this.

Solves #979.
I did a bit of investigation into why errors weren't being logged on esp.mit.edu, it looks like this line was failing with an IOError, no such file or directory.

Also, I think this was the only use of `settings.LOG_FILE` (it's not a Django built-in setting, as far as I can tell). Should we get rid of references to that too?

(Django has a different setting for configuring logging; see https://docs.djangoproject.com/en/1.4/ref/settings/#std:setting-LOGGING)