Enable Libravatar by default in new installations (#3287)

Pre-existing installs still have it default to off.
This is consistent with OpenID setting.

modules/setting/setting.go

@@ -1038,7 +1038,7 @@ func NewContext() {
 		GravatarSource = source
 	}
 	DisableGravatar = sec.Key("DISABLE_GRAVATAR").MustBool()
-	EnableFederatedAvatar = sec.Key("ENABLE_FEDERATED_AVATAR").MustBool()
+	EnableFederatedAvatar = sec.Key("ENABLE_FEDERATED_AVATAR").MustBool(!InstallLock)
 	if OfflineMode {
 		DisableGravatar = true
 		EnableFederatedAvatar = false