Allow experimental flags to be enabled per user

[matrixbot]

This issue has been migrated from #15234.

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It would be useful to allow experimental flags to be enabled per user, to allow those with matrix.org accounts to test features with their real account without having to enable it for all matrix.org users.

Not all experimental features can be enabled per-user, e.g. experimental login flows wouldn't make sense to enable per-user (since the requester hasn't logged in yet!).

My proposal would be to store these in the database and add admin APIs. The rationale being that then we can (potentially) allow users to self-serve opting into experimental features.