Ignore imported documents in version control

Successor to #2277 via @fstanis and @pbakaus: rather than just having an easy to oversee comment block at the head of imported files the files should actually be ignored by version control.

This could either be achieved by dynamically writing the .gitignore file by using the information in platform/config/imports/spec.json. Though I fear that might cause additional confusion.

Another option would be to try to import all files into a common location and then ignore this location once - that's the solution I prefer and would implement.