[FEATURE] Require that users be logged in on lower environments

[dlh01]

This is a practice that we usually implement and could be generalized here.

[mboynes]

Interesting idea. Historically, I've used Bachhuber's dead-simple Site Secrets plugin for this. What I like most about a plugin approach is that I can disable it briefly if needed. I feel like that would be a must-have for this feature: to be able to disable it without committing code (relates to #16).

This begs a larger question: for features like this, where an existing plugin offers a fine solution, should Alleyvate duplicate that functionality?