Key bindings; disable on disconnect.
Key bindings; disable on disconnect.

This reinstates the feature whereby we disable the content script when
we lose contact with the background page, e.g., on upgrade.

From my investigations, this doesn't appear to be absolutely necessary.
Nevertheless, it's cleaner like this.

philc/vimium,decaffeinate-examples/vimium,poacher2k/vimium,mrmr1993/vimium,mrmr1993/vimium,willsALMANJ/vimium,poacher2k/vimium,decaffeinate-examples/vimium,mrmr1993/vimium,smblott-github/vimium,gdh1995/vimium,philc/vimium,decaffeinate-examples/vimium,gdh1995/vimium,smblott-github/vimium,poacher2k/vimium,willsALMANJ/vimium,smblott-github/vimium,gdh1995/vimium,willsALMANJ/vimium,philc/vimium