Add exemption for lang param in locale middleware.

There may be some URLs which have other uses for a
'lang' query parameter. This allows you to set such URLs as exempt.

This is mostly for bedrock, as we need the /firefox/download/ page
to accept a ?lang= parameter that refers to the language of the
Fx build you wish to download. That param for that page has existed
for a long time and we're trying to port it from the PHP side
to bedrock.