Support regex blacklisting domains/URLs for URL previews

[jellykells]

Currently it's possible to configure the matrix-media-repo URL previewer to blacklist ranges of IP addresses, but many hosts do not have a static IP. Synapse allows regex blacklisting of both domains and URL paths.

One workaround is to add an entry in the hosts file on the matrix-media-repo host that resolves a domain to a blacklisted IP, but this has limitations such as requiring an entry for each domain and subdomain due to lacking support for wildcards or regex.

References: Synapse URL preview URL blacklist docs