Devices can get out of sync between Synapse and MAS

[sandhose]

The provisioning and deletion of devices is not exactly reliable (see #2625), which means sometimes the device list in Synapse gets out of sync with the one in MAS.

The long term solution may be to correctly model Matrix devices in MAS instead of relying solely on the scope for OAuth sessions and on a field in the compatibility sessions.

In the meantime, we should at least have a way to force a re-sync of the whole device list for a user, and maybe even regularly do that for all users.