This PR makes the expired/invalid tenant impersonation tokens get
deleted instead of just aborting with 403.
The PR also adds a command (ClearExpiredImpersonationTokens) used like
`php artisan tenants:purge-impersonation-tokens`. As the name suggests,
it clears all expired impersonation tokens (= tokens older than
`UserImpersonation::$ttl`).
Resolves#1348
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Co-authored-by: Samuel Štancl <samuel@archte.ch>