API: Enforce section=new constraint when using 'redirect' mode

Finish implementation of the 'redirect' edit API parameter.  This flag
should only be used as a convenience when adding a new section, appending,
or prepending.  Any other usage must be done using the long-cut (fetch page,
determine if it is a redirect, follow redirects and edit the target page's
content).

This patch takes apart the "EditConflict_redirect" test, because it no
longer makes sense--unless you dear reader can figure out how to force an
edit conflict when only adding a section?

Bug: 24330
Change-Id: Ie3f7273c18e156da1e50e1a36aac2e5341710982
Merge "API: Enforce section=new constraint when using 'redirect' mode"