Allow to work with CSR protection

src/main/java/org/jenkinsci/plugins/ghprb/GhprbCrumbExclusion.java

@@ -0,0 +1,33 @@
+package org.jenkinsci.plugins.ghprb;
+
+import hudson.Extension;
+import hudson.security.csrf.CrumbExclusion;
+
+import javax.servlet.FilterChain;
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+
+/**
+ * Excludes {@link GhprbRootAction} from the CSRF protection.
+ * @since 1.28
+ */
+@Extension
+public class GhprbCrumbExclusion extends CrumbExclusion {
+
+    @Override
+    public boolean process(HttpServletRequest req, HttpServletResponse resp, FilterChain chain)
+            throws IOException, ServletException {
+        final String pathInfo = req.getPathInfo();
+        if (pathInfo != null && pathInfo.startsWith(getExclusionPath())) {
+            chain.doFilter(req, resp);
+            return true;
+        }
+        return false;
+    }
+
+    public String getExclusionPath() {
+        return "/" + GhprbRootAction.URL + "/";
+    }
+}