Enable plugins to create config stores for repository config

Therefore we have to
- add an API to create stores for repository ids, not only for
  repositories,
- make v1 properties available in scm-core
- make sure that properties are extracted from repositories before the
  update step of a plugin runs (this is done by sorting the update steps
  in a way so that "core" update steps are executed before plugin update
  steps with the same version)

scm-webapp/src/main/java/sonia/scm/update/UpdateEngine.java

@@ -32,12 +32,20 @@ public class UpdateEngine {
   private List<UpdateStep> sortSteps(Set<UpdateStep> steps) {
     LOG.trace("sorting available update steps:");
     List<UpdateStep> sortedSteps = steps.stream()
-      .sorted(Comparator.comparing(UpdateStep::getTargetVersion).reversed())
+      .sorted(
+        Comparator
+          .comparing(UpdateStep::getTargetVersion)
+          .thenComparing(this::isCoreUpdateStep)
+          .reversed())
       .collect(toList());
     sortedSteps.forEach(step -> LOG.trace("{} for version {}", step.getAffectedDataType(), step.getTargetVersion()));
     return sortedSteps;
   }
 
+  private boolean isCoreUpdateStep(UpdateStep updateStep) {
+    return updateStep instanceof CoreUpdateStep;
+  }
+
   public void update() {
     steps
       .stream()