Use gist URLs instead of RawGit in React docs (#2385)

RawGit is being shut down (eg Oct 2019, https://rawgit.com/) and this change uses the direct links to the files as served from gist.github.com.  The same files are being served, just that the links are only dependent on GitHub's gist service.

Author: davidjb

content/docs/add-react-to-a-website.md

@@ -69,13 +69,13 @@ The first two tags load React. The third one will load your component code.
 
 Create a file called `like_button.js` next to your HTML page.
 
-Open **[this starter code](https://cdn.rawgit.com/gaearon/0b180827c190fe4fd98b4c7f570ea4a8/raw/b9157ce933c79a4559d2aa9ff3372668cce48de7/LikeButton.js)** and paste it into the file you created.
+Open **[this starter code](https://gist.github.com/gaearon/0b180827c190fe4fd98b4c7f570ea4a8/raw/b9157ce933c79a4559d2aa9ff3372668cce48de7/LikeButton.js)** and paste it into the file you created.
 
 >Tip
 >
 >This code defines a React component called `LikeButton`. Don't worry if you don't understand it yet -- we'll cover the building blocks of React later in our [hands-on tutorial](/tutorial/tutorial.html) and [main concepts guide](/docs/hello-world.html). For now, let's just get it showing on the screen!
 
-After **[the starter code](https://cdn.rawgit.com/gaearon/0b180827c190fe4fd98b4c7f570ea4a8/raw/b9157ce933c79a4559d2aa9ff3372668cce48de7/LikeButton.js)**, add two lines to the bottom of `like_button.js`:
+After **[the starter code](https://gist.github.com/gaearon/0b180827c190fe4fd98b4c7f570ea4a8/raw/b9157ce933c79a4559d2aa9ff3372668cce48de7/LikeButton.js)**, add two lines to the bottom of `like_button.js`:
 
 ```js{3,4}
 // ... the starter code you pasted ...
@@ -195,7 +195,7 @@ npx babel --watch src --out-dir . --presets react-app/prod
 
 Don't wait for it to finish -- this command starts an automated watcher for JSX.
 
-If you now create a file called `src/like_button.js` with this **[JSX starter code](https://cdn.rawgit.com/gaearon/c8e112dc74ac44aac4f673f2c39d19d1/raw/09b951c86c1bf1116af741fa4664511f2f179f0a/like_button.js)**, the watcher will create a preprocessed `like_button.js` with the plain JavaScript code suitable for the browser. When you edit the source file with JSX, the transform will re-run automatically.
+If you now create a file called `src/like_button.js` with this **[JSX starter code](https://gist.github.com/gaearon/c8e112dc74ac44aac4f673f2c39d19d1/raw/09b951c86c1bf1116af741fa4664511f2f179f0a/like_button.js)**, the watcher will create a preprocessed `like_button.js` with the plain JavaScript code suitable for the browser. When you edit the source file with JSX, the transform will re-run automatically.
 
 As a bonus, this also lets you use modern JavaScript syntax features like classes without worrying about breaking older browsers. The tool we just used is called Babel, and you can learn more about it from [its documentation](https://babeljs.io/docs/en/babel-cli/).