IA-5415: Fix org unit types infinite recursion

[Phil-V]

What problem is this PR solving?

The OrgUnitType api could throw RecursionError when sub_unit_types or allow_creating_sub_unit_types references were forming a loop. This PR adds extra validation to prevent these cycles and makes the API more resilient against existing loops.

Related JIRA tickets

IA-5415

Changes

• Add validation for allow_creating_sub_unit_types to prevent loops.
• Fix some api responses that were likely intended to have depth of 1 but were recursively loading related OrgUnitTypes.
• Make the api more resilient against loops if they somehow exist (admin, import scripts, etc.).

How to test

Field validation:

• Go to the OrgUnitTypes page (Org units > Configuration > Organization unit type)
• Edit some org unit types and set their "Sub org unit types to display/create" fields and try to create infinite loops (A -> B, B -> A for example).
• The api should return a validation error.

Api resilience against loops:

• Through the django admin, set up some OrgUnitTypes that reference each other in a loop through their sub_unit_types and allow_creating_sub_unit_types attributes.
• Check the following endpoints for errors:
  GET /api/v2/orgunittypes/?fields=id,name,sub_unit_types
GET /api/v2/orgunittypes/?fields=id,name,allow_creating_sub_unit_type
GET /api/v2/orgunittypes/1/hierarchy/ (with the pk of a type involved in the loop)

Print screen / video

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Notes

• Opted not to add validation to the django admin. I was concerned this could create situations where it becomes impossible to fix or modify the OrgUnitTypes without shell/db access.
• Nested DynamicFieldsModelSerializer still use the fields param from the request to override the default fields, creating the potential for recursion. Judging from how the serializers were defined (with a list of fields that don't include the subtypes), this was likely not to be the intended response. As a workaround,ignore_dynamic_fields was added to suppress that behavior.

Doc

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[bmonjoie]

LGTM 👍
Great job