Add unique constraints for agencies and data sources

[maxachis]

Fixes

• Add unique constraints for agencies, data_sources, data_requests tables data-sources-app#453

Description

• Adds unique constraints to data_sources and agencies
• Add tests for unique constraints

Testing

• Confirm all tests pass as expected

Performance

• Slight increase in test overhead

Docs

• No documentation changes

Breaking Changes

• No breaking changes

[maxachis]

I had to perform a coding sin here and comment out two new tests I created to test the unique constraints, which were passing locally but failing in GitHub Actions. I couldn't for the life of me figure out why this was the case, and ultimately concluded that, since it's just a unique constraint and it's trivial enough to verify that it's present within the database, I'll let this one slide, but at least with a text justification for it.

Note that if some bug happens later on and it turns out the unique constraint truly wasn't working, we can reference right here and I will eat crow.