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/download_resource.py

@@ -37,9 +37,8 @@ class DownloadResource(Resource):
         self.media_repo = media_repo
         self.server_name = hs.hostname
 
-        # Both of these are expected by @request_handler()
+        # this is expected by @wrap_json_request_handler
         self.clock = hs.get_clock()
-        self.version_string = hs.version_string
 
     def render_GET(self, request):
         self._async_render_GET(request)

synapse/rest/media/v1/preview_url_resource.py

@@ -58,7 +58,6 @@ class PreviewUrlResource(Resource):
 
         self.auth = hs.get_auth()
         self.clock = hs.get_clock()
-        self.version_string = hs.version_string
         self.filepaths = media_repo.filepaths
         self.max_spider_size = hs.config.max_spider_size
         self.server_name = hs.hostname

synapse/rest/media/v1/thumbnail_resource.py

@@ -44,7 +44,6 @@ class ThumbnailResource(Resource):
         self.media_storage = media_storage
         self.dynamic_thumbnails = hs.config.dynamic_thumbnails
         self.server_name = hs.hostname
-        self.version_string = hs.version_string
         self.clock = hs.get_clock()
 
     def render_GET(self, request):

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):