Local storage is leaked every time a DBC is loaded for a new drive

[jyoung8607]

Cabana retains a local copy of the DBC used to interpret a given drive, per drive, indefinitely. This accumulates over time and eventually makes Cabana stop working until the browser cache is cleared, or the local storage entities are nuked by hand in the developer console.

It's nice to have all those workspaces held for later reference, particularly if edits are in progress, but a mechanism is needed to gracefully react to quota exhaustion.