Re-add the posibility to purge all submissions

Problem/Motivation

In #2937608 the ability select which form submissions should be purged was introduced.

While the module automatically sets all existing webforms to be purged on install/update, all new webforms are not added to purge list automatically.

As a consequence it's not possible to have a workflow where you are relying on all submissions to be purged after some time automatically, without manually managing the list of webforms to be purged and adding new to it.

Proposed resolution

I suggest adding a new option "Purge all submissions" on the admin page, which negates the purge list and ensures that all webform submission a purged regardless.