Import RDA DCS plan into any fully compatible template

[hmpf]

Possibly a little big, but very clear steps. For met.no and others who want machine-actionable plans.

• Add a table RDADCSCompatible or something in the rdadcs app that has a foreign key to template. If a template is reachable from the new table it is assumed that it is compatible. Should probably also check the RDA DCS keys of the compatible templates
• Update the import helper functions/classes to act on a non-hardcoded template reachable from RDADCSCompatible.
• Update CLI, admin to import to an explicitly specified template reachable from RDADCSCompatible.
• Add an endpoint (or change the existing rdadcs import endpoint) to import into an explicitly specified compatible endpoint.

Maybe have an rdadcs-tree to put the "import to rdadcs compatible" endpoint in, instead of overloading the template and plan endpoints even more?