Resolve "Allow importing and exporting of a file field"

Closes #408 (closed)

It was already possible to export all the applications of a group, to later import then again or to use it as a template. The only downside was that the file field was not supported because it was more work to export the files related to the file field. Support for this field has now been added. When exporting a group, a ZIP file containing all the related files is now also exported. More information how this works exactly and the slightly updated way of creating a template can now be found in the updated docs/development/create-a-template.md file.

Some testing pointers

• I added a file field to the applicant tracker template. So running the sync_templates command should work and after that, the template should have a CV file field in the applicants table that should have a couple of exmaple CV's.
• It should be possible to use the export_group_applications command for a group that has a database table containing a file field.
• It should be possible to import the exported files using the import_group_applications command and the files should still exist.