File storage refactoring, adding far more flexibility to Django's file handling. The new files.txt document has details of the new features.
This is a backwards-incompatible change; consult BackwardsIncompatibleChanges for details.
Fixes #3567, #3621, #4345, #5361, #5655, #7415.
Many thanks to Marty Alchin who did the vast majority of this work.
