mjolnir frequently creates new devices

[richvdh]

It seems like mjolnir creates a new device every few hours. This is problematic because (a) it means that we accumulate hundreds of active sessions on the @abuse account, but more pertinently because (b) it is incompatible with the need to ignore insecure devices for e2ee messaging, per matrix-org/matrix-spec-proposals#4153.

We need to figure out why mjolnir is doing this (possibly it happens when it gets a 503 from the homeserver?), and make it not.

[turt2live]

This also causes Mjolnir to lose its admin bit, which is more problematic operationally.

Logs indicate that it's when the server 503's the bot, or when there's an introspection error.