Add an "Amplify" option to Become A Member page

[sahadeva]

• [x] Include a field in the Become a Member page template for Amplify code:
• [x] If that field has a code in it, display a download link to get the Amplify app and instructions for how to use it in the Become a Member page (should become step 4)
• [ ] (optional): Include how documentation should be updated to reflect this change

[sah]

Instructions text from Ted:

We all want to be in the habit of doing the weekly TeamName actions. But it's hard to do if we're each acting alone. Join us on the Amplify mobile app built by Indivisible SF. We update it each week with our list of actions, and you can view call scripts and call members of congress right from the app. You can also see the stream of actions taken by other members, and press the "cheer" button to cheer them on.

Join us on the app:

1. Install the app for iPhone (https://getamplify.org/iphone) or Android (https://getamplify.org/android)
2. Press 'Create New Account' to sign up
3. Enter the Test Team invite code: 101-202-303
4. Take an action & cheer others on!