Reset CSRF token to null when logging out from a server

With the current state of the extension, the CRSF token acquired on the first
invocation to authenticate is not cleared on logout and thus is getting reused
by follow-up authentication calls resulting in CSRF errors.

src/main/java/qupath/lib/images/servers/omero/OmeroWebClient.java

@@ -554,6 +554,7 @@ public void logOut() {
 			timer.cancel();
 			timer = null;
 			username.set("");
+			this.token = null;
 		} catch (IOException e) {
 			logger.error("Could not logout.", e.getLocalizedMessage());
 		}