Ensure PPIDs are retained on page deletion + restoration

[JeroenDeDauw]

If the MediaWiki Page ID changes in the deletion+restoration process (I don't know if it does), then the PPID should be linked to the new ID. This might already work, but we need to verify it, add an integration test, and potentially fix it.

Somewhat related:

• Ensure PPIDs are retained on page move #71

[malberts]

At least the most straightforward case of deleting a page immediately after creating it does not change the PPID (or MW page ID). See #32

However, there might be cases where other very specific actions inbetween the create-delete-undelete actions cause the PPID to change. In those cases I expect it would be because the page got a new ID, which in turn would cause our lookup to return nothing.