Detect internal URLs in menu using config.siteUrl

This is so that menu items defined in Ghost don't get mistakenly
processed as external sites

src/components/common/Navigation.js

@@ -1,6 +1,7 @@
 import React from 'react'
 import PropTypes from 'prop-types'
 import { Link } from 'gatsby'
+import config from '../../utils/siteConfig'
 
 /**
 * Navigation component
@@ -17,6 +18,8 @@ const Navigation = ({ data, navClass }) => (
         {data.map((navItem, i) => {
             if (navItem.url.match(/^\s?http(s?)/gi)) {
                 return <a className={navClass} href={navItem.url} key={i} target="_blank" rel="noopener noreferrer">{navItem.label}</a>
+            } else if (navItem.url.slice(0, config.siteUrl.length) == config.siteUrl) {
+                return <Link className={navClass} to={navItem.url.slice(config.siteUrl.length, navItem.url.length)} key={i}>{navItem.label}</Link>
             } else{
                 return <Link className={navClass} to={navItem.url} key={i}>{navItem.label}</Link>
             }