Remove unnecessary vendor prefixes

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Adapt copyright Rake task for Sass sources

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Use single-line Sass comments for copyright header

Sass single-line comments are preferable as they are removed from
CSS output (thus fewer KB's for end users).

We do not convert copyright headers for entry-point stylesheets
(default.css, legacy.css, scm.css).

See also de29c354.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #2070 from opf/feature/stylesheet-cleanup

Stylesheet cleanup