Get rid of drf_yasg

[mofr]

In the current implementation to generate OpenAPI it's required to have Django URLConf: ⚠️ generator = OpenAPISchemaGenerator(api_info, urlconf=django_urlconf) ✔️ The new implementation will be based on winter routes directly.

To do:

• [x] Create new version of OpenAPI spec in form of dataclasses (preferably OAS version 3)
  • Option 1: https://github.com/kuimono/openapi-schema-pydantic
  • Option 2: Hand-crafted OpenAPI specification in form of dataclasses
  • Option 3: apispec
• [x] Implement GET endpoint to serve spec file without drf_yasg (using winter route or Django directly)
• [x] Migrate existing users
• [x] Delete drf_yasg-related code