Extension should consistently warn user whenever changing workspace to multiroot

[jcreedcmu]

It's possible some actions are permitted --- possibly specifically adding a database from LGTM --- without checking whether the workspace is single-root. The consequence of this is, I think, that the user can unintentionally wipe out their query history when the extension reinitializes.

[aeisenberg]

Alternatively, we can keep query history across restarts. That's more complicated, but also more valuable.