Legacy URL tool

[ss23]

From http://open.silverstripe.org/ticket/1993

It would be good to have a system for dealing with Legacy URLs in the CMS.

Perhaps each page in the CMS can have a Legacy URL field. If that URL is visited, the user would be permanently redirected to the new page.

If we implemented the "optional director rule" system for multi-lingual sites - having a director rule that is applied only in some circumstances - we could use that to implement a LegacyURLController that was checked for every URL, but only executed if the URL passed matched something in the LegacyURL column of SiteTree?.

This could be implemented as an add-on module quite easily - a decorator on SiteTree? and an extra, high priority director rule.

@sminnee