Show loading gif while uploading a work.

[shortland]

Over 2Mb has shown to sometimes take awhile... So let's show a gif to show that stuff is loading...

[shortland]

instead: show toast:

"Document is uploading..."
(this message will disappear when done)

Requires me to add new function to https://github.com/TLTMedia/Marginalia/blob/master/js/Modules/Toast.js
w/timeout being configurable to existing method or w/just timeout = 0 (it'll show forever)
then wipe the notification using some toast method - when upload is done.