Fix one-to-one relation in User/File

Signed-off-by: Dominik Pataky <git@netdecorator.org>
Move filehash create method; replace registered_files with User.files

Signed-off-by: Dominik Pataky <git@netdecorator.org>
Filehash MD5->SHA512; move hash generation into class; add is_valid()

I try to put all methods needed by classes into them, but some are still
needed from outside. Maybe @static might be a solution.

File.is_valid() checks if the storage holds a file with the hashstring
as name.

Signed-off-by: Dominik Pataky <git@netdecorator.org>
Add FileToken database entity

Signed-off-by: Dominik Pataky <git@netdecorator.org>
Add token download route; fix relation link file/owner

The route /dl/<token> takes a token string and additionally the
filename and sends the file from the storage to the client.
There is no counter or related stuff implemented yet.

Signed-off-by: Dominik Pataky <git@netdecorator.org>
Add token display in upload table

Signed-off-by: Dominik Pataky <git@netdecorator.org>