Never delete pages that have user modifications in them

Here's what happened to me on myxwiki.org:

• I upgraded my wiki (massol.myxwiki.org) using the DW
• It removed the blog app I was using!! (I guess this is because the new ui-common doesn't have the blog app anymore but that's a big bug from a user POV)
• It removed my customization I had in Blog.WebHome (the blog title and other settings).

I believe that the EM should never remove a page that has custom user changes in them.

More generally it should not have deleted my blog app without at least asking. That's a big blocker.