New api endpoint to submit error reports to developers

[raffis]

Describe the solution you'd like

New writeonly api endpoint POST /api/v2/feedbacks
The error reports are either sent directly to a remote service in which case balloon only acts as a proxy.

There must be no form of authentication since that may prevent one of sending an error report.

The endpoint must accept a zip within the body and a message. Most likely as
multipart/mixed.

This should likely be implemented as Balloon.App.Feedback.

Additional context

The desktop client already uses a remote feedback endpoint. This must change to the new internal endpoint.

A feedback functionality may also be implemented in the web interface.
Errors >= 500 may get reported directly.