fix: make room directory correct when using a homeserver with explicit port
Server names are allowed to contain ':' to specify a port, see https://matrix.org/docs/spec/appendices#server-name
User ids on the other hand are not allowed to contain ':', even
historical user ids, see https://matrix.org/docs/spec/appendices#historical-user-ids
Therefore we can use change the regex to make sure the localpart is not
allowed to contain ':'.