Only handle image blobs on front and backends

[miketaylr]

Filing this as a "future work" issue.

In #1248 we allow the frontend to handle blobs OR base64 screenshot data, but send it all up to the server as base64 strings. It would simplify code to just handle blobs everywhere, since you can treat blobs the same as File objects.

In order for this to happen, we need to make changes in a few places: frontend, server code, add-ons, and the Fennec Nightly reporter code.

Potentially there's perf wins here. Interesting read that's only partially relevant: Esri/offline-editor-js#206