Add option to open tabs as discarded

[globau]

To do this we'll need to call browser.windows.create with a single url, wait for the window to be created, then use browser.tabs.create to add the other tabs with a discarded property.

Need to determine if the issue with the promise returned by browser.windows.create never returning has been fixed.

[globau]

Need to determine if the issue with the promise returned by browser.windows.create never returning has been fixed.

What's happening is I'm creating the windows from within stash.js, which is run within the popup window's context. When the window closes, the script is terminated so the promise has no where to send the result to.

Instead the popup should be messaging the background script, which in turn handles window creation, etc.