Add config option to specify admin contact or support page (MSC1929)

[6543]

If an admin contact or support page is set in the configuration,

there is already a admin_contact, it might be usefull to add extra config that then uses (MSC1929) to announce who is in charge of an homeserver.

synapse should then serve this well-known url!

refs:

• https://github.com/matrix-org/matrix-federation-tester/issues/157
• https://github.com/the-draupnir-project/Draupnir/pull/150
• https://spec.matrix.org/v1.10/client-server-api/#getwell-knownmatrixsupport
• https://github.com/ruma/ruma/pull/1751

help to fight spam and report spam accounts to the homeserver admins etc ...

[6543]

-> https://github.com/element-hq/synapse/blob/develop/synapse/rest/well_known.py