Show the dismiss X for facets applied as ?column__exact=value

[cycsmail]

Closes #2011.

The filter form always redirects to ?column__exact=value (filters_should_redirect defaults the op to exact), but ColumnFacet only looked for ?column=value when deciding if a facet value was selected, so a facet applied that way rendered as a plain link with no X to remove it. That matches the screenshot on the issue: Sole Source was applied by clicking the facet, so it gets an X, while Contract Type came from the filter form and doesn't. The spaces and punctuation in the value turned out to be a red herring.

Facet values now match either form of the querystring, and the toggle link removes whichever key is actually in the URL. One thing I left alone: when a __exact filter is present, the links for the other values in that facet still append ?column=value next to it, same as before this change.

Added a regression test, the 19 tests in tests/test_facets.py pass.

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