Enable `/find/` endpoint

[TimJentzsch]

Relevant issue: Closes #225.

Description:

In #223 I added the /find/ endpoints to get submissions/transcriptions by their URL, but I forgot to actually enable it for usage. This PR fixes this problem, so that we can actually use it.

It also improves/fixes the Swagger documentation for the endpoint.

Checklist:

• [x] Code Quality
• [x] Pep-8
• [x] Tests (if applicable)
• [x] Success Criteria Met
• [x] Inline Documentation
• [ ] Wiki Documentation (if applicable)