Extension is unresponsive when sending many pages

[haroldtreen]

Current Behavior

When a user with a slow connection or many tabs uses the service, clicking the Download button makes the extension unresponsive.

Expected Behaviour

When Download is clicked, the steps before talking to the server (grabbing the HTML, making the request) should be shown within the loader.

Info

EpubPress is receiving messages from all the tabs containing all their HTML. So if you have 20 pages open, each with 10 Mb of HTML, that's 200 MB+ of RAM that will be needed just to transfer the HTML.

The solution to this problem could be both showing a loader to make things look more responsive, but also chunking the data.