Handle html returned as ajax response

The responses to ajax requests in qr.js and citation.js are not expected
to be html. It seems there are cases where titles that do not exist in
the crossref & datacite databases resolve to the DOI destination instead
of returning a failed metadata notice. Treat these cases as failed
fetches.
Optional use of custom resolver for all services

Configure usage of custom resolver for autolinker, bubble, context menu,
and omnibox from options.
New option to choose default tab for omnibox

Select whether omnibox entry should use the current tab, a new
background tab, or a new foreground tab.