Avoid excessive remote requests on oEmbed lookups

Avoid excessive remote requests on oEmbed lookups

I noticed that each time a notice was accessed it'd do a remote lookup
with HEAD (and continue despite 404 etc.) and then another attempt to
download the resource. If this wasn't successful new attempts would be
made for each loading of the resource, which is extremely resource
intensive.

Whenever we can say "it's been n seconds since the last attempt" we
could probably enable this again - or just manually reload remote
thumbnails (as part of the StoreRemoteMedia plugin etc.)