Add `find` endpoint

[TimJentzsch]

Relevant issue: Closes #186.

Description:

This will move most of the functionality of the /find command from Buttercup to Blossom. This has several advantages:

• It simplifies the code.
• It's a lot easier to test.
• We can use the functionality in other apps, such as Bubbles

In particular, this endpoint takes any Reddit URL (ToR submission, partner submission or transcription; new or old Reddit) and returns the corresponding Blossom objects.

Future Work

Also accepting claim and done URLs. They can't be translated to the submission URL directly, so they are a bit harder to handle.

Checklist:

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