Implement a redirector at Special:Flow

Special:Flow/post/<UUID> redirects to the canonical URL of a PostRevision,
and Special:Flow/workflow/<UUID> redirects to that of a workflow.
Special:Flow shows a form where the type (post or workflow) can be
selected and a UUID can be entered.

Bug: 63510
Change-Id: I8222b6a2d9f7a45645e8bc12fe1c7a893a93fbf6