Keep duplicated filterable list params when paginating

[wpears]

Closes #2199 from over 4 years ago!

The problem here stemmed from the use of request.GET.items(), which returns only the last value when multiple params have the same key, by design.

See https://docs.djangoproject.com/en/3.1/ref/request-response/#django.http.QueryDict.items

Changing to .lists() and updating the logic a bit to handle the resulting lists stops params from being dropped.
Also, filtering out the 'page' param prevents the other bug here which was repeated page params in the querystring.

[willbarton]

This is fine... I'm kind of surprised that we're diving into request.GET in a template, though. That should really be done in a Django form that hands off the appropriate values. But so should a lot of other places where we're using request params.

[anselmbradford]

@wpears

Do me a favor…

• pull this branch and run it
• go to http://localhost:8000/blog
• expand the filter and check "At the CFPB" and "Director's notebook"
• click "Apply filters'
• scroll to the bottom of the results and enter "3" in the pagination component.
• click "Go"

Do you get this error?

[wpears]

@anselmbradford I did encounter this (once) but I don't think it's related to this PR. I'm looking into the source of the error.

[wpears]

Almost certainly related to #5984

[wpears]

Yeah confirmed, will merge this and PR a different fix (the error is local only)