Queue items like multiple Scene URLs in scan queue or multiple forced scrapes of sites

[moToroTor]

Similar to #398, sometimes a user wants to add a few scenes by URL. If you have, say, 3 scenes, and paste them into the manual scan field, and click scan, the second scene (because the first is still scanning) will be accepted but not actually scanned.

This request is to enable queuing of actions, so if you do multiple things, they actually occur in order. This would also be nice to do for the Scrapers section, if you want to run scrapes of multiple scrapers, you can't, the action will seem to complete but not actually do anything.