Add Android feedback collection endpoint

[aekeus]

• Add an endpoint to collect feedback from a mobile device
• Store feedback in Mongo
• Add script to remove feedback entries by id or date
• Add tests

Implements #110

[kkuehlz]

Let’s prioritize https://github.com/brave/vault-updater/pull/117 and rebase this on top of that branch. I don’t want to introduce more code that needs security upgrade