[v2] Misc fixes necessary to get gatsbyjs.org (mostly) working (#3975)

* WIP

* webpack 3 is much more strict about not mixing es6 and commonjs

* Update gatsby-plugin-glamor for babel 7 + add @babel/preset-flow

* Add uglifyjs-webpack-plugin as a dependency so we're getting the latest version w/ es6 support

* MISC

* Change reference page queries to not rely on path info being in the ID

* Switch to using pageContext instead of pathContext

* Ignore the compiled index.js file

* Update peerDependencies for gatsby v2

* Update yarn.lock

* Add peerDependencies to gatsby-link

* Remove test code

* Update gatsby-link dependency

* Fix some tests

package.json

@@ -24,6 +24,6 @@
     "@babel/runtime": "^7.0.0-beta.38"
   },
   "peerDependencies": {
-    "gatsby": "^1.0.0"
+    "gatsby": ">1.0.0 || >2.0.0-alpha"
   }
 }