Add 'purge' tasks

At NYC Camp this past weekend, I heard several requests for such a feature. It's been referred to in other issues before, such as #1611536: Site cannot be deleted after failed install. It can be a major AegirWTF. Currently, the manual process of removing the cruft left over from partially-installed or partially-deleted sites is a form of tribal knowledge passed on through IRC, and in the documentation. We should endeavour to find a way to automate (at least parts of) this procedure.

This should be considered an unsafe task, and require a confirmation dialog. It should also (optionally) only appear in the task list when there's already indication that something is broken about the site. For example, a failed install or delete task. We should be fault-tolerant, so that the purge task itself doesn't fail if it cannot accomplish a given step. But it should definitely give feedback if it can't remove something. For example, say a site failed to delete because there are files that are owned by 'www-data', and that 'aegir' doesn't have permission to delete. In such a case, we should throw a warning into the task log indicating what manual intervention may be required to complete the purge.

Similarly, we should provide such a 'purge' ('force delete' would also be a reasonable name) task for platforms.

The steps required are detailed in the wiki pages I've linked to above.