core (xmpp), component file sharing, plugin XEP-0363: enabled_features + HTTP Upload:

add a mechanism to explicitely enable some features in components, and use it to enable
HTTP Upload only if it's explicitely enabled.

sat/core/xmpp.py

@@ -1067,6 +1067,7 @@ class SatXMPPComponent(SatXMPPEntity, component.Component):
                 )
             )
 
+        self.enabled_features = set()
         self.identities = [disco.DiscoIdentity("component", "generic", C.APP_NAME)]
         # jid is set automatically on bind by Twisted for Client, but not for Component
         self.jid = component_jid