don't bother getting file size if file is missing
make DDRFile.make_thumbnail a static method

I forget exactly why I decided to do this.  Probably so it could
be used with access files as well as master files.
settings for thumbnail and access file geometry, options, etc
use thumbnail geometry,options settings in add_file()
DDRFile.make_access_file function
add DDRFile.access_rel field and setter functions

access_rel points to an access file if present.
The setter function adds access_abs and access_size if access_rel
is present and points to something.
make_access_file() in webui.tasks.add_file; rm make_thumbnail()
use file.access_rel for thumbnailing rather than file.path

Access file is a reliably smaller size than the original files,
and so is much less likely to cause performance problems and
crashes.
added entity_add_access task
added view/form/template/etc for add-access-file function/task
make_access_file complains if source file does not exist
make_access_file complains if can't create access file

Before, it was falling through this set of if-thens and returning
the default 'unknown' result in certain cases.
make_access_file uses only first layer of multilayer images

PDFs and certain TIFs can contain multiple layers.
Imagemagick accomodates this by creating multiple output files.
The DDR code didn't know what to do with multiple images, though,
which caused access file and thumbnail generation to fail.