Expose popup code for A/B tests

[virginiacc]

Adds email popup template to base template and exposes code on window so it can be used in a/b tests

Changes

• Add popup template to base template
• Expose EmailPopup on window

Testing

• Check out branch and run grunt
• Clear local storage:
  • localStorage.setItem('oahPopupShowNext', '')
• Initialize an email popup on any OAH page (except home page) from console:
  • var popup = new window.EmailPopup(); popup.init(); popup.showPopup();
• Clear local storage and check that popup does not appear after scrolling down process close page
• Check that popup does appear after scrolling 50% on another journey page

Review

Screenshots

Todos

Checklist

• [ ] Changes are limited to a single goal (no scope creep)
• [ ] Code can be automatically merged (no conflicts)
• [ ] Code follows the standards laid out in the front end playbook
• [ ] Passes all existing automated tests
• [ ] New functions include new tests
• [ ] New functions are documented (with a description, list of inputs, and expected output)
• [ ] Placeholder code is flagged
• [ ] Visually tested in supported browsers and devices
• [ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)