Now that the cloud platform exposes a system-settings endpoint for the
user's selected organization, switch `UserStore` over to it and drop the
local database persistence. The server already returns the current
selected organization for `get_authenticated_user()`.

set_current_organization becomes a optimistic, with the in-memory
selection updated immediately. hen a background task POSTs to the system
settings endpoint.

Stale current_organization_id rows from older installs are left dangling
— harmless and not worth a migration. This is the second step in moving
organization selection to cloud; the first added the
update_system_settings client method.

Part of CLO-716

Release Notes:

- N/A

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Co-authored-by: Marshall Bowers <git@maxdeviant.com>