Handle link prefix and trail in the DOMPostProc

* We now support link prefix
* We now handle link prefix and link trail in the DOM post-processor, because
  it's the simplest place to do it that has access to the env.conf.wiki values
  for the link prefix/trail regexes.

Change-Id: Ie7726d7a9d0f5e1a25398b059bd5aaa1324c9b3d
Merge "Handle link prefix and trail in the DOMPostProc"