BackoneJS update to 1.0.0 and CDN usage
Update BackboneJS to 1.0.0 and use CDN at the same time (also for
underscore)
Upstream commit: mboinet/ttrss-mobile@c94ab7b7054e743f8660aacc661373f35bf5fb1c

Style in an external CSS
It is one more file to fetch, but cleaner. The plan is to use a
minimizer later.
Upstream commit: 8d5509f76be64c82c8f4eadc91cc055a5be99898