make DDRFile objects for Entity even if files absent

Users need to be able to see/edit metadata for files even if the
actual files are not present in the current repo.
functions for indicating presence/absence of files, access files

There's currently no way of running os.path.exists in a Django
template.
mute file download and accessfile generate btns if files absent
don't crash if media_target is None
Merge branch 'master' of https://github.com/densho/ddr-local