Backend functionality for adding custom attachments in applications

[emsoraffa]

No description provided.

[emsoraffa]

So far whats been implemented for ApplicationFileAttachment -> serializer, model and viewset. Still some issues with uploading attachments that needs to be fixed.

In general what remains to be done is:

1. Fixing the serializer/viewset so that uploads work properly
2. Adding frontend functionality s.th files are retrieved for existing applications
3. Recruitment position should be where file types, filesize etc is defined
4. validation, i.e is the url actually a url, and is the file an image/pdf etc