fix: Fix jetty crashing when redirecting HTTP to HTTPS

Jetty 9.3 changed the `setHandler` on the ServletContextHandler to no
longer automatically detect SecurityHandler, SessionHandler, etc. It
simply passes on the setHandler request to the ContextHandler class
(with a warning logged). So make sure to explicitly use the method
`setSecurityHandler` to set the ContraintSecurityHandler responsible
for the http -> https redirection.

src/main/java/com/gitblit/GitBlitServer.java

@@ -453,7 +453,7 @@ public class GitBlitServer {
 			sh.setConstraintMappings(new ConstraintMapping[] { cm });
 
 			// Configure this context to use the Security Handler defined before
-			rootContext.setHandler(sh);
+			rootContext.setSecurityHandler(sh);
 		}
 
 		// Setup the Gitblit context