Move the User-Agent styleesheet to a CSS file instead of a Rust string.

… as suggested in
https://github.com/mozilla/servo/pull/1015#issuecomment-25755957

Also add a comment on the `border-width` work-around.
auto merge of #1017 : SimonSapin/servo/external-UA-css, r=jdm

… as suggested in https://github.com/mozilla/servo/pull/1015#issuecomment-25755957

Also add a comment on the `border-width` work-around.