Implement a feature flag system

## Priority We need it for the MVP, since the MVP should only include working / tested features ## Context Currently, all the features are shown to everybody at any time. This isn't ideal as some features are not finished, and might give the wrong impression to potential users or investors. ## Solution Some tools: - https://www.growthbook.io (I've used it before, it does the job) - https://openfeature.dev (more DIY, never used) - https://www.flagsmith.com (seems similar to growthbook)