Email signup popup - Githubissues

Adds email signup popup on /process/prepare, /process/compare, and /process/explore

Additions

Check out branch & run grunt
Navigate to http://localhost:8000/owning-a-home/process/prepare/, http://localhost:8000/owning-a-home/process/explore, or http://localhost:8000/owning-a-home/process/compare/ and scroll down the page. Popup should appear after you've scrolled about halfway down.
Reload the page or navigate to another page. Popup should not appear after scrolling down page.

[ ] 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)