Objects API "contract" feature - improvements

[Viicos]

Follow up of #3688:

• Backend validation of the variables mapping (Phase 4). Requires the refactor (see Rethink admin Javascript to save a form #3941) to land first.
• Display backend validation errors back to the frontend:
  • For specific variables, error should be displayed in the variables tab
  • An error should also be displayed if required targets are not mapped. The issue is we are constrained by the DRF error format (strings), and this probably requires a more complex structure.
• target-paths/ endpoint: handle connection errors to the OT API. Currently returns 500.
• Display a message when no target path is available? Currently only displays the ------ empty option, which can be confusing.
• Objects API reg.: Handle object types where allowGeometry is false #4408
• To figure out once we get more feedback: the order in which mappings are applied matters (e.g. you can map an object to path > to, and then a string to path > to > otherpath)
• Use React select for the target-path dropdown
• Add a popup when switching from v2 to v1, saying all the configured variable mappings will be deleted: [#4031] Add a confirm popup when switching to v2 options #4054