Support OpenGraph video embeds (#4897)

* Support OpenGraph video embeds

It's not really OpenGraph, it's twitter:player property, but it's
not OEmbed so that fits. For example, this allows Twitch clips to
be displayed as embeds.

Also, fixes glitch-soc/mastodon#135

* Fix invalid OpenGraph cards being saved through attaching and
revisit URLs after 14 days