Avoid accessing the superglobals directly

With CodeClimate now setup, I went thru some of the items reported. I suggest going thru the security issues first.

https://codeclimate.com/github/Automattic/jetpack/issues?category=security

For starters it seems instead of accessing Superglobals directly we should be using `filter_input` or something similar. This comes up quite a bit.

http://php.net/manual/en/function.filter-input.php

Start with the CodeClimate security report linked in the issue and inventory the direct superglobal accesses it identifies across Jetpack. Review PHP's filter_input documentation, then define completion as addressing the reported accesses consistently and verifying that the security findings are resolved.

Written by the indexing model from the issue text.