Escape commas in urls in whitehall mappings file

Some urls in whitehall mappings contain commas. Although a comma is a
valid character in a url path[1] the commas cause problems with the rest
of the munge/redirector processing pipeline.

In particular this addresses issues that have arisen with processing the MOJ mappings:

https://www.pivotaltracker.com/story/show/47517313

[1] http://tools.ietf.org/html/rfc3986#section-3.3