Allow wikis to be redirected when deleting

[edg2s]

As mentioned in https://github.com/MatmaRex/patchdemo/issues/54#issuecomment-647591436 and also from experience, when creating a wiki that updates an old patchset, it can be annoying to update links already shared to the new wiki.

Now that we also have to the patchdemobot on Phabricator, it is also impossible for those posts to be updated by other users.

We could add an option when deleting a wiki to specify a wiki to redirect to.