replace QuartzScheduler with a more lightweight scheduler

The new scheduler is based on the cron-utils package and uses the same cron syntax as quartz.
CronScheduler was mainly introduced, because of a ClassLoader leak with the old Quartz implementation.
The leak comes from shiros use of InheriatableThreadLocal in combination with the WorkerThreads of Quartz.
CronScheduler uses a ThreadFactory which clears the Shiro context before a new Thread is created (see CronThreadFactory).

scm-webapp/src/main/java/sonia/scm/schedule/PrivilegedRunnableFactory.java

@@ -0,0 +1,29 @@
+package sonia.scm.schedule;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import sonia.scm.web.security.AdministrationContext;
+
+import javax.inject.Inject;
+import javax.inject.Provider;
+
+class PrivilegedRunnableFactory {
+
+  private static final Logger LOG = LoggerFactory.getLogger(PrivilegedRunnableFactory.class);
+
+  private final AdministrationContext context;
+
+  @Inject
+  PrivilegedRunnableFactory(AdministrationContext context) {
+    this.context = context;
+  }
+
+  public Runnable create(Provider<? extends Runnable> runnableProvider) {
+    return () -> context.runAsAdmin(() -> {
+      LOG.trace("create runnable from provider");
+      Runnable runnable = runnableProvider.get();
+      LOG.debug("execute scheduled job {}", runnable.getClass());
+      runnable.run();
+    });
+  }
+}