feat: implement archival mode to prevent event deletion

[oyetanishq]

TODO: Archival Mode: no deleting of events

• Add new events.archivalMode configuration flag to strfry.conf.
• When this flag is enabled, it prevents the deletion of events.

[hoytech]

Interesting, thank you! I think archival mode might need a bit more thinking/specification though. Your technique is nice and simple, it just doesn't delete events. However, the queries will still return these events to external users, which may not be desired. For example, if you queried someone's contact list, you'd get a bunch of old useless revisions.

Also, I believe there are some edge-cases in the replaceable events where the algorithms assume there is at most one event matching a replacement tag.

I was thinking of maybe moving events to an isolated archive table, instead of deleting them, but I haven't fully thought this through as yet.