Warn when first syncing a database containing data to a server account containing data

[dstillman]

If the local DB is empty and has never been synced, or if there's no server data, no problem.

"Are you sure? This will merge this library with the items on the server, possibly resulting in duplicates."

Also warn when attempting to sync with a different account from the one last used with the current database.

[dstillman]

Milestone 2.0 Beta 3 deleted