[JENKINS-46925] Do not show the Action if the thing is not configurable.

The action was still showing up on projects that could never have
configuration added.  When you tried to add configuration you would get
the User X is missing the Item.Configure permission, but its just not
good UX, so hide the action in case System can not configure the thing
(ie it can never have config files)

src/main/java/org/jenkinsci/plugins/configfiles/folder/FolderConfigFileAction.java

@@ -10,6 +10,8 @@
 
 import javax.servlet.ServletException;
 
+import org.acegisecurity.context.SecurityContext;
+import org.acegisecurity.context.SecurityContextHolder;
 import org.jenkinsci.lib.configprovider.ConfigProvider;
 import org.jenkinsci.lib.configprovider.model.Config;
 import org.jenkinsci.lib.configprovider.model.ContentType;
@@ -31,6 +33,7 @@
 import hudson.model.Action;
 import hudson.model.Item;
 import hudson.model.Job;
+import hudson.security.ACL;
 import hudson.security.Permission;
 import hudson.util.FormValidation;
 
@@ -53,7 +56,22 @@ public AbstractFolder<?> getFolder() {
 
     @Override
     public String getIconFileName() {
-        return folder.hasPermission(Item.EXTENDED_READ) ? ConfigFilesManagement.ICON_PATH : null;
+        // whilst you may be able to read configuration of a ComputedFolder they are not configurable, so the link should never be shown.
+        boolean hasPerm = folder.hasPermission(Item.EXTENDED_READ);
+        if (hasPerm) {
+            // check that the thing can store configuration (ie it is ultimately configurable)
+            // need to do this as System as a user with EXTENDED_READ but not configure should still see this.
+            // TODO use ACL.as when baseline is 2.14+
+            final SecurityContext impersonate = ACL.impersonate(ACL.SYSTEM);
+            try {
+                hasPerm = folder.hasPermission(Item.CONFIGURE);
+            }
+            finally {
+                SecurityContextHolder.setContext(impersonate);
+            }
+        }
+
+        return hasPerm ? ConfigFilesManagement.ICON_PATH : null;
     }
 
     /**