Federated reports

This MR adds support for federated reports in a way that is Mastodon-compatible.

Differences between Pleroma and Mastodon federated reports:

• Pleroma federated reports have stable IRI-style IDs (https://pleroma.example.com/activities/...). I asked Mastodon to add the same, so that we can send Update activities about the disposition of the report down the line.

• Pleroma reports are NOT anonymized, this 'feature' is annoying for many reasons. If someone really wants it, we can add support for anonymization later (by allowing the reporting user to be configured).