When a module rejects a piece of media we end up trying to close the
same logging context twice.
Instead of fixing the existing code we refactor to use an async context
manager, which is easier to write correctly.
During the migration the automated script to update the copyright
headers accidentally got rid of some of the existing copyright lines.
Reinstate them.
There are two situations which were previously not properly checked:
1. If the requested URL was replaced with an oEmbed URL, then the
oEmbed URL was not checked against url_preview_url_blacklist.
2. Follow-up URLs (either via autodiscovery of oEmbed or to pre-cache
images) were not checked against url_preview_url_blacklist.
