Ability to expire older megolm keys from local storage.

We're getting to the point where some folks have >50MB of megolm keys, which becomes very unwieldy to export/import and maintain.

I've had a request to be able to locally expire megolm keys older than a given age (although could keep them on the online backup for safe keeping).

Start by locating the Element client code that stores local megolm keys and the online-backup flow. Determine how an age-based expiration setting would interact with local storage and backup retention; done means older local keys can be expired without removing their backed-up copies.

Written by the indexing model from the issue text.