simplify scm-manager bootstrap

We have simplified the scm-manager bootstrap process, by replacing the
ServletContextListener call hierarchy with much simpler ModuleProviders.
We have also removed the parent injector. Now we create always a new
injector. If something goes wrong in the process of injector creation,
we will show a nicely styled error page instead of stacktrace on a white
page.

scm-webapp/src/main/java/sonia/scm/boot/InjectionLifeCycle.java

@@ -0,0 +1,60 @@
+package sonia.scm.boot;
+
+import com.google.inject.Binding;
+import com.google.inject.Injector;
+import com.google.inject.Key;
+import sonia.scm.CloseableModule;
+import sonia.scm.Default;
+import sonia.scm.EagerSingletonModule;
+import sonia.scm.ServletContextListenerHolder;
+
+import javax.servlet.ServletContext;
+import javax.servlet.ServletContextEvent;
+import java.util.Optional;
+
+class InjectionLifeCycle {
+
+  private final Injector injector;
+
+  InjectionLifeCycle(Injector injector) {
+    this.injector = injector;
+  }
+
+  void initialize() {
+    initializeEagerSingletons();
+    initializeServletContextListeners();
+  }
+
+  void shutdown() {
+    destroyServletContextListeners();
+    closeRegisteredCloseables();
+  }
+
+  private void initializeServletContextListeners() {
+    ServletContextListenerHolder instance = injector.getInstance(ServletContextListenerHolder.class);
+    ServletContext context = injector.getInstance(Key.get(ServletContext.class, Default.class));
+    instance.contextInitialized(new ServletContextEvent(context));
+  }
+
+  private void initializeEagerSingletons() {
+    findInstance(EagerSingletonModule.class).ifPresent(m -> m.initialize(injector));
+  }
+
+  private void closeRegisteredCloseables() {
+    findInstance(CloseableModule.class).ifPresent(CloseableModule::closeAll);
+  }
+
+  private void destroyServletContextListeners() {
+    ServletContextListenerHolder instance = injector.getInstance(ServletContextListenerHolder.class);
+    ServletContext context = injector.getInstance(Key.get(ServletContext.class, Default.class));
+    instance.contextDestroyed(new ServletContextEvent(context));
+  }
+
+  private <T> Optional<T> findInstance(Class<T> clazz) {
+    Binding<T> binding = injector.getExistingBinding(Key.get(clazz));
+    if (binding != null) {
+      return Optional.of(binding.getProvider().get());
+    }
+    return Optional.empty();
+  }
+}