Purge both https and http urls from Facebook cache on save

[jonom]

If CMS is accessed through https connection but http version of page is normally shared, cache may not be cleared (need to test)

[jonom]

This should happen in the background, perhaps as a queued job, so it doesn't slow down page saves.

[jonom]

Perhaps include a config option (e.g. DualProtocol) for this. Some sites don't have SSL certs and some are only accessible through https, so we don't want to add a redundant http request in those cases.