Update page IDs in revision JSON when copying

Fixes #837

When copying a page, all of its revisions are also copied. The revisions
are JSON encoded and include ID fields which point back to the original
page object. These IDs weren't being updated when the revision was
copied to a new page causing the editor to break when that new page is
edited.
Create new revision when copying pages

This serves two purposes:
 * It makes sure update_attrs gets applied to the latest revision so the
   changes are reflected in the editor
 * It bumps the last_revision_created_at value so the new page gets
   ordered as if it was just created
Added user attribute to Page.copy

This is used for setting the owner of the new pages and the user for the
new revision that gets created
Order by created_at when getting first revision
Changed test to use eventmoderator user
Merge branch 'issue/837' of https://github.com/kaedroho/wagtail into kaedroho-issue/837
Merge branch 'kaedroho-issue/837'
Merge branch 'master' of github.com:torchbox/wagtail