Improve disconnect UX

[raucao]

From litewrite/litewrite#293

I see two issues there:

1. When there's unsynced local data present, the library should warn the user about it when a disconnect is attempted, with the option for the user to cancel the disconnect and re-auth the app again (or get online and sync data, depending on what's the reason).
2. It might make sense to give the user the choice to keep all synced local data when disconnecting, i.e. put it back in anonymous local storage. It's certainly debatable if the benefits of forcing this choice on every disconnect outweigh the drawbacks in UX. So let's discuss. :)

[aw]

Thanks for discussing this.

There's also a use-case for a storage provider locking a user account or removing the account entirely. In that situation, the user's data becomes remotely inaccessible, but since there's a local copy, it should be possible to sync it to another location.