Port findings platform over to new pattern

[hellais]

This is about taking https://github.com/ooni/backend/blob/master/api/ooniapi/incidents.py and porting it over to make use of fastapi.

Here are the specs for how it should be:

• Service should be named oonifindings
• Service should be 100% identical to old incidents
• Service should follow the template of oonirun, but without postgresql and instead clickhouse
• Service should make use of the incidents tests to ensure it's working as intended. We should add more tests to reach 100% code coverage.
• Service should include metrics using prometheus
• We should also add an integration test that performs actions on the incidents backend-fsn API and compares the results with the new one (they should behave identically).
• Should setup clickhouse as a fixture in pytest by assuming the clickhouse server binary is installed on the system and available in PATH. It should start the server in a way that's it's throw away for the testing session, see how pytest-postgresql does this: https://github.com/ClearcodeHQ/pytest-postgresql

Once we have an initial iteration that's 100% compatible to the previous one we can work on a v2 version of the API that looks nicer, follows our more up to date API patterns and is backed by postgresql.

Keep an eye on this issue for docs on best-practices to come: #817

[jbonisteel]

First deploy with current api compatibility.
We will make a separate issue where we make new end points based on the feedback from the PR review

[jbonisteel]

We will move this to prod once we have reduced AWS costs a bit