🚨Warning message on purge (#1984)

# Description

We had a user purging the space instead of instances, leading to data
loss. This is a stop-gap solution to try to avoid this happening again.
Once #1977 and #1978 are merged, I will add a better solution for it in
the reimplementation of this command.

## Changelog

- [x] Patch
- [ ] Skip

## cdf

### Improved

- When running the `cdf purge space` command, the user will now get an
extra warning.

## templates

No changes.
