MMS and URLs are not working due to enable_authenticated_media

Need to enable authenticated media for MMSes and URLs to continue to work.

https://element-hq.github.io/synapse/latest/usage/configuration/config_documentation.html#enable_authenticated_media

Seems like this option is going away soon so I'll need to decide what to do next: https://github.com/element-hq/synapse/issues/17950