Previously we would use messaging to open the next link with either
a loaded content script in the current tab or a new tab if the tab
can't load a content script. This makes it simpler and also has the
added benefit of no longer requiring the permission to access
all websites.
The number of URLs queued is restricted to ~60 because the queue is stored as a single item and is therefore is restricted to 8k.
Instead store each URLs as a separate item with a name 'qi'+number and gather them into a single array when loading/storing.