This commit introduces a new preview property for link content items.

 * `httpsAccessible`: Whether or not a link is available over HTTPS

It also fixes an issue in the embeddable check by passing in a fake
user agent.
Added a testcase that marks http-only URLs as  + some minor cleanup
#748 - minor code cleanup
Merge pull request #748 from simong/pp-links

PP - Check if a http link is available on https (and vice versa)