Fix #1277 use rsync -c option as all output is rewritten

Because Pelican always rewrites all output, the mtimes always change, so
rsync would always transfer all the files which defeats the purpose of
rsync. The '-c' option (--checksum) compares the checksums rather than
mtimes.
Really fix #1311 by declaring CSS overrides as !important

this is needed because on RTD the common hosted theme stylesheets get added
after the overrides.