matrixsynapse: use conf.d snippets to reduce conffile conflicts

Hi, during a backports update end of last year I was warned about conffile conflicts and was worried about losing existing config / missing out on improvements. I delved into the homeserver.yaml today and it matches the file as of buster after applying the plinth post_install plus some reformatting. I found #1754 (closed) which explains why it was eventually upgraded fine without intervention, but this left the latest maintainer's configuration in homeserver.yaml.dpkg-dist which does have some improvements.

I'm not sure how long conf.d has been there, but e.g. the server_name is set in /etc/matrix-synapse/conf.d/server_name.yaml, so I suggest popping at least max_upload_size and password_providers into their own config file rather than editing the homeserver.yaml. I'm not sure how the listeners bind_addresses can be done, it would need more testing, but if possible that would remove the conffile prompts entirely and allow inheriting the new debian defaults in v1.7.