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/test/java/sonia/scm/boot/SingleViewServletTest.java

@@ -1,6 +1,5 @@
 package sonia.scm.boot;
 
-import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.api.extension.ExtendWith;
 import org.mockito.Mock;