Add Google Tag Manager

[mbertrand]

Integrate Google Tag Manager into the app

For reference:
https://github.com/mitodl/ocw-course-hugo-theme/pull/26/files
https://github.com/mitodl/bootcamp-ecommerce/pull/198/files
https://github.com/mitodl/micromasters/pull/4328/files

[mbertrand]

Related: #1009