Mini "control panel" / CLI commands for cleanup of unused elements

[bseigel]

Ability to see which entry types, sections, fields, categories, assets, etc. are in use, and delete them en masse if they're not. Simple interface w/checkboxes in the control panel, similar tools in CLI.

Why?

Sometimes we need to clone a single or multisite install, and then excise a bunch of stuff. Doing this item by item is really tedious.