Change URL preview setting from bool to a mode, and add support for an allowlist

debian/postinst

@@ -84,9 +84,6 @@ allow_check_for_updates = true
 # Enable the display name lightning bolt on registration.
 enable_lightning_bolt = true
 
-# Allows clients to request a URL preview
-allow_url_preview = false
-
 # Servers listed here will be used to gather public keys of other servers.
 # Generally, copying this exactly should be enough. (Currently, Conduit doesn't
 # support batched key requests, so this list should only contain Synapse
@@ -99,6 +96,13 @@ trusted_servers = ["matrix.org"]
 #
 # [0]: https://docs.rs/tracing-subscriber/latest/tracing_subscriber/filter/struct.EnvFilter.html#directives
 #log = "..."
+
+# possible URL preview modes:
+#  None: previews disabled
+#  All:  previews for any URL allowed
+#  Allowlist: only domains in \`url_preview_allowlist\` are allowed
+url_preview_mode = "None"
+url_preview_allowlist = ["google.com", "youtube.com", "www.youtube.com"]
 EOF
     fi
     ;;