core: feature flags

[danielhochman]

Description

Add an endpoint to the backend that serves feature flags. Initially this can be a true/false per variable. Later we will want to add per-user and per-group targeting for features, so that a feature developer can pilot the feature in a real environment without exposing it to all users.

On the frontend, we'll need a library that can easily flip workflow packages on and off based on the values. The values will need to block loading.

Complexity [S/M/L]: M

Wireframes, Mockups, or Screenshots (if applicable)
N/A

[danielhochman]

Ideally we would also have developer tools on the frontend, or at least javascript console commands, that would allow you to toggle the values at will in the current window.