replace embedded MailChimp form with Customizer controls for headline, blurb, and button

[benlk]

Changes

This pull request makes the following changes:

• Removes the homepage Mailchimp signup form; replaces it with a JS-powered Constant Contact one. The JS comes from Sarah's email in https://secure.helpscout.net/conversation/1213856192/5823?folderId=2730118#thread-3480078682

Why

For https://github.com/INN/umbrella-inndev/issues/165#issuecomment-655135189 and https://secure.helpscout.net/conversation/1213856192/5823?folderId=2730118

Testing/Questions

Features that this PR affects:

-

Questions that need to be answered before merging:

• [x] Is this PR targeting the correct branch in this repository?
• [ ] Is this deployed to staging?
• [ ] How can styles be improved here?
• [ ] Are we concerned about what this form looks like if the subscribe form does not load? Does this need fallback content inside the div?

Steps to test this PR:

1. Visit the homepage of the INN site and scroll down to "Stay Up To Date"

[joshdarby]

Is this deployed to staging?

I don't see it on http://inndev.staging.wpengine.com/

How can styles be improved here?

I'll leave that one to @MirandaEcho

Are we concerned about what this form looks like if the subscribe form does not load? Does this need fallback content inside the div?

It might be a good idea to have some sort of error message there if the form doesn't load.

[benlk]

Feedback:

• [x] remove form js/HTML
• [x] add button that links to same place as the "Subscribe" button in the header of inn.org
• [x] change heading to "Stay INN the Loop"
• [x] change subheading to "News from INN and our members in our INNovation newsletter. Delivered weekly."

[benlk]

There is no specific setting for the header link to the Subscribe button; it is in a Custom HTML widget.

Therefore, this PR now implements Customizer fields for changing the headline, blurb, button text, and button link for the newsletter subscribe area of the homepage.

[benlk]

Approved for prod.