Display process bar from cloud exporter in regular exporter sidebar

[jywarren]

@divyabaid16 I wanted to see if you'd like to give this a try, building on the amazing work @sashadev-sky and others have done to make the cloud export function work.

In https://mapknitter.org/maps/pvdtest/, try holding shift, then dragging from lower right to upper left, and you should see images get selected. Then you should see a down arrow button appear in upper left; if you click it, it'll start an export process, but with no feedback to the user. See #782 for how this is hooked up!

Can we display the ongoing export in the client side in just JavaScript, using the same progress bar UI we have for in-app exports?

I think this may take some mockups, but since we can now make multiple exports at once (using cloud export) we could start to show exports in a list in the exports tab, instead of just having one at a time. Then each could have a set of downloads that it offers.

What do you all think of this? Thinking in coordination with #659 #693 #526

[jywarren]

#413 #552 too!