Only attempt rewriting https or http URLs.

Fixes #330, exception processing about:newtab.
Exclude safebrowsing URL from rewriting.
Merge pull request #475 from jsha/no-about

Only attempt rewriting https or http URLs.
Slovak Goverment Institutions

This are Slovak institutions listed in Central Government Portal
https://www.slovensko.sk/sk/institucie
Adbusters.org
Merge pull request #479 from MilanKral/SlovakGoverment

Slovak Goverment Institutions
Merge pull request #480 from MilanKral/Adbusters.org

Adbusters.org
Implement a downgrade & duplicate whitelist.

Ultimately, we hope to remove these lists (or significantly reduce them).
Merge pull request #481 from semenko/restrict-downgrade-and-dup

Implement downgrade & duplicate whitelists
Merge pull request #478 from jsha/safer-browsing

Exclude safebrowsing URL from rewriting.