webfonts: fixes

- Treat WOFF as binary files so that git does not perform newline normalization.
- Replace corrupt Heuristica files with Source Serif Pro &mdash; italics are [almost in production](https://github.com/adobe/source-serif-pro/issues/2) so I left Heuristica Italic which makes a good pair with SSP. Overall, Source Serif Pro is I think a better fit for rustdoc (cc @TheHydroImpulse). This ought to fix #15527.
- Store Source Code Pro locally in order to make offline docs freestanding. Fixes #14778.

Preview: http://adrientetar.legtux.org/cached/rust-docs/core.html

r? @alexcrichton
Since we're changing this line, wouldn't it be better if the fallback fonts were also serif? (Just `"Source Serif Pro", serif` would be enough, IMO)