Set Server header in SynapseRequest

(instead of everywhere that writes a response. Or rather, the subset of places
which write responses where we haven't forgotten it).

This also means that we don't have to have the mysterious version_string
attribute in anything with a request handler.

Unfortunately it does mean that we have to pass the version string wherever we
instantiate a SynapseSite, which has been c&ped 150 times, but that is code
that ought to be cleaned up anyway really.

synapse/rest/media/v1/upload_resource.py

@@ -41,7 +41,6 @@ class UploadResource(Resource):
         self.server_name = hs.hostname
         self.auth = hs.get_auth()
         self.max_upload_size = hs.config.max_upload_size
-        self.version_string = hs.version_string
         self.clock = hs.get_clock()
 
     def render_POST(self, request):