Parameterize app name in web interface

[svenaas]

Changes proposed in this pull request:

• Front end uses of "Federalist" or "Pages" are parametrized based on environment, including
  • Web application text
  • Web page title
  • Web page meta tags
  • Web page body DOM class (added to facilitate parametrizing CSS background image)
  • Application logo images
  • Messages, alerts, and warnings
  • Email subject lines
  • Support email address (pages-support@cloud.gov / federalist-support@gsa.gov) (except in uaaInvite.js which is only relevant to Pages)
• Numerous changes in testing to make things work locally (usually Pages), on Circle (Federalist), and on Concourse (Pages).

This is work toward #3709

security considerations

None

[svenaas]

I want to do a round of spot-checking on this locally in the browser before I ask for a code review.

[davemcorwin]

Assuming nothing breaks, this looks ok to me. We can play around in the staging envs as well.

It's not worth changing here, but it's worth evaluating whether or not we need the app name at all in a lot of the client-facing messaging, we probably could have just dropped it in a bunch of places. 🤷