Support retrying of uploads

[jakearchibald]

It feels like something we shouldn't do by default, as making the same "POST" request twice may have side effects.

Perhaps we should provide an opt-in to this? Something that says "My server is coded in a way that prevents double-posting. Please retry POSTs".

+@beverloo @johnmellor

[jakearchibald]

This might be redundant if we can standardise resumable uploads.

[TheRook]

background fetch cannot do post, retrying should be fine... but you cannot trigger a background fetch within the serviceworker - you need an attached client and perhaps use postMessage which isn't trivial to do.