We often have cases where we want to create a link that will go back to the

same page, but with a single different querystring parameter. Traditionally

these links have been hard-coded, and doing so will drop all the other things

in the querystring. We do have a helper utility for doing what we want in

python (get_url_params_except), which is used in some places.
This change adds a new templatetag, querystring_with, that will build a new

querystring that has everything in the current one, but with the value of a

single variable changed.

Used this with other changes.