make footer blurb about subscription editable in the cms

[tanyabouman]

- Summary

I was using this template and I needed parts of the footer to be editable in the CMS. I thought that might be something useful to contribute upstream, and maybe I can get some feedback on the design.

I added the footer fields to the home page and then used those variables in the footer partial, so that they are editable on the home page.

- Test plan

deploy

- Description for the changelog

Make subscription text in footer editable in CMS

- A picture of a cute animal (not mandatory but encouraged)

[tanyabouman]

Thanks for the feedback! I put the footer fields into data/footer.json