Force HTTPS protocol

[matbrady]

@Henryhankdc @tedw let's discuss what and where an update should be included. Either in the docs or actual code updates.

[tedw]

Links to docs:

• https://pantheon.io/docs/pantheon-yml#enforce-https--hsts
• https://pantheon.io/docs/redirects

I think we should also include info on redirecting https://live-mysite.pantheonsite.io to the primary domain.