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/update/MigrationWizardModuleProvider.java

@@ -0,0 +1,27 @@
+package sonia.scm.update;
+
+import com.google.inject.Injector;
+import com.google.inject.Module;
+import sonia.scm.boot.ModuleProvider;
+import sonia.scm.update.repository.XmlRepositoryV1UpdateStep;
+
+import java.util.Collection;
+import java.util.Collections;
+
+public class MigrationWizardModuleProvider implements ModuleProvider {
+
+  private final Injector bootstrapInjector;
+
+  public MigrationWizardModuleProvider(Injector bootstrapInjector) {
+    this.bootstrapInjector = bootstrapInjector;
+  }
+
+  public boolean wizardNecessary() {
+    return !bootstrapInjector.getInstance(XmlRepositoryV1UpdateStep.class).getRepositoriesWithoutMigrationStrategies().isEmpty();
+  }
+
+  @Override
+  public Collection<Module> createModules() {
+    return Collections.singleton(new MigrationWizardModule());
+  }
+}