Allow upgrading unmanaged local DBs

[glatterf42]

As noticed by @meksor, it would be good to allow initializing locals DBs. Currently, this is only possible for managed DBs. This PR also allows upgrading/initializing unmanaged local DBs. Though please check: settings.manager_url and settings.secret_hs256 have default versions, so we could also write:

    utils.echo(
        f"Establishing self-signed admin connection to '{settings.manager_url}'."
    )
    manager_conf = ManagerConfig(
        str(settings.manager_url),
        SelfSignedAuth(settings.secret_hs256),
        remote=False,
    )
    platform_list = manager_conf.list_platforms() if settings.managed else settings.toml.list_platforms()

which might safe us from writing a test to cover these lines specifically, but introduces slight overhead costs for running ManagerConfig().__init__ even for unmanaged instances. Not sure which is preferable here.

[meksor]

I think this is good as it is, thanks!