Add api endpoints for device save restructuring

This MR adds explicit resources for:

• customfields
• device attachments
• platform attachments
• (and now there are also tests for all of them)

So that they can be handled as JSON:API resources themselves. Currently the "old" way to include them in the devices & platforms is still possible, but they now should be seen as deprecated. The changes here also don't break the old API (which removing of the nested attributes in devices & platforms whould do), so I also stay with the v1 version number.