Add UrlQuotes linter

Add linter which checks for literal URLs (i.e. URLs which are not
contained within quotes). This is a bizarre feature of CSS that doesn't
seem to offer much convenience and instead introduces inconsistency by
attempting to treat URLs as first-class entities.

Change-Id: I171d3e0fad3e37cc281206cddfe51ac158ff5e3b
Reviewed-on: https://gerrit.causes.com/32825
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Tony Wooster <tony.wooster@causes.com>