Make reviewer.enableByEmail inheritable

This change makes the config inheritable, adds it to PutConfig
and adapts the test code and docs where needed.

The GWT UI is adapted such that the config can be set using the UI
control.

Change-Id: I7b4a52eebbb69c44c1095a6a4ea3e402d1eb4206

gerrit-server/src/main/java/com/google/gerrit/server/project/PutConfig.java

@@ -154,6 +154,10 @@ public class PutConfig implements RestModifyView<ProjectResource, ConfigInput> {
         p.setState(input.state);
       }
 
+      if (input.enableReviewerByEmail != null) {
+        p.setEnableReviewerByEmail(input.enableReviewerByEmail);
+      }
+
       if (input.pluginConfigValues != null) {
         setPluginConfigValues(ctrl.getProjectState(), projectConfig, input.pluginConfigValues);
       }