Report progress when submitting forms

[NoTuxNoBux]

It's a small thing, but it would be nice if there was a progress bar/line of some sort when a form save is happening. Usually the submit is fast enough for the UI to be responsive, but in case of file uploads you are just staring at the submit button in the loading state and have no clue how far along the upload is.

If it's annoying in scenarios where the save doesn't take long, it could optionally be shown only if at least one file upload field is present, and/or when the submit is taking over the minimum of n seconds.

(I'm trying to tie this in manually at the moment, with these examples as guidance for Apollo, but it's quite painful since there appears to be no direct way to hook into Keystone's GraphQL queries on the client side.)