[gatsby-source-medium] fetch users and publications (#3623)

* Add support for fetching a users payload along with publications from Medium

* Update gatsby-source-medium readme, add note for @ for usernames

* Undo any changes to `links` variable

packages/gatsby-source-medium/README.md

@@ -23,6 +23,8 @@ plugins: [
   },
 ];
 ```
+###### Note
+Remember that if you are fetching a user, prepend your username with `@`.
 
 ## How to query
 

packages/gatsby-source-medium/src/gatsby-node.js

@@ -32,15 +32,29 @@ exports.sourceNodes = async ({ boundActionCreators }, { username }) => {
     const result = await fetch(username)
     const json = JSON.parse(strip(result.data))
 
-    const { posts } = json.payload
-    const collectionKeys = Object.keys(json.payload.references.Collection)
-    const userKeys = Object.keys(json.payload.references.User)
-
-    const importableResources = [
-      userKeys.map(key => json.payload.references.User[key]),
-      posts,
-      collectionKeys.map(key => json.payload.references.Collection[key]),
-    ]
+    let importableResources = []
+    let posts = {} // because `posts` needs to be in a scope accessible by `links` below
+
+    const users = Object.keys(json.payload.references.User)
+      .map(key => json.payload.references.User[key])
+    importableResources = importableResources.concat(users)
+
+    if (json.payload.posts) {
+      posts = json.payload.posts
+      importableResources = importableResources.concat(posts)
+    }
+
+    if (json.payload.references.Post) {
+      posts = Object.keys(json.payload.references.Post)
+        .map(key => json.payload.references.Post[key])
+      importableResources = importableResources.concat(posts)
+    }
+
+    if (json.payload.references.Collection) {
+      const collections = Object.keys(json.payload.references.Collection)
+        .map(key => json.payload.references.Collection[key])
+      importableResources = importableResources.concat(collections)
+    }
 
     const resources = Array.prototype.concat(...importableResources)
     resources.map(resource => {