Add a simple feature flag system

[danieladugyan]

Description

We often develop large features on a single long-lived feature branch. This discourages continuous integration and instead often leads to an annoying merge process once the feature has been developed. A feature flag system could help with this - a way to temporarily wrap new features with some conditional logic that checks whether the feature should be visible to users. That way we could merge incrementally without exposing unfinished features.

Solution

Implement feature flagging. Maybe all feature flags should be enabled by default in development and disabled by default in production? Ideally, there would be a way for us to enable features for ourselves in production as well (but not for all users) so we can try them out.