Resource-priorities

Prépare http://www.w3.org/TR/resource-priorities/
https://github.com/marienfressinaud/FreshRSS/issues/316
https://github.com/marienfressinaud/FreshRSS/issues/313
Continue https://github.com/marienfressinaud/FreshRSS/issues/188
https://github.com/marienfressinaud/FreshRSS/commit/e45357a91b9aa47d5b7ead14c174dc7c98ab9926