Add list of currently applied feature flags to performance bar

Problem to solve

Feature flag usage will continue to increase as we attempt to increase developer velocity; more code is going out behind flags, and interaction between these flags is often untested or surprising. Having a means to discover which flags are currently being applied to the user session (or rendered page?) even if incomplete, would be useful.

Intended users

• developers working on GitLab features
• users attempting to identify and report bugs or unexpected behaviors

Further details

This would contribute to the utility of the Performance Bar as a feature.

Proposal

flipper has an internal API for this already, so we'd want to check current_user, the current project, the groups, etc. If this proves expensive, we could instead move this to an asynchronous API endpoint behind a button that the user might click to engage with.

Permissions and Security

Documentation

Performance Bar documentation will need to be updated to reflect the new feature.