Inbox deletion: warn users more warnier

[moggers87]

Very very occasionally we get a user who has deleted an inbox despite our warnings and submits a support request asking for an inbox to be restored to their account.

I don't really want to remove or change how deleting an inbox currently works: that will confuse users and remove a feature that was actually being asked for.

There are a couple of things we could do to make users think twice:

• ask them to type "I understand" in a text box
• offer an "empty and disable" (i.e. soft delete) button next to the delete button

Current warning: