Dynamic filter form population can alter presentation of ChoiceFields elsewhere in the UI

[glennmatthews]

Environment

• Nautobot version (Docker tag too if applicable): 1.6.0rc1

Steps to Reproduce

1. Log in to Nautobot
2. In one tab, open the /dcim/devices/ list view, open the Filter modal, and switch to the Advanced tab.
3. In another browser tab, open the "run" view for a Job that defines a ChoiceVar. Observe that the ChoiceVar is correctly represented as a single-value Select2 widget in the UI.
4. In the first tab, in the Filter modal, select "Rack Face" in the first column, then "exact" in the second column.
5. Refresh the second tab.

Expected Behavior

No change on page refresh

Observed Behavior

The ChoiceVar now gets incorrectly rendered as a multi-value Select2 widget.

This is due to incorrect code in get_filterset_parameter_form_field() which is inadvertently modifying the forms.ChoiceField class directly instead of only changing a single instance of that class.

[glennmatthews]

Thank you to the many folks who helped identify, reproduce, and troubleshoot this issue, including @Dav-C, @jtdub, @joewesch. @matt852.

[glennmatthews]

Workaround to temporarily "unbreak" the Job form after this issue is encountered:

• Navigate again to /dcim/devices, open the Filter modal, and switch to the Advanced tab.
• Select "Has a primary IP" and then select "exact"
• Refresh the job tab.

This is because there's a second code path that also incorrectly modifies forms.ChoiceField when making a different kind of selection from the advanced filter form; we're just lucky in that this code path modifies it to somewhat closer to the original, desired behavior of the form field. Both code paths will be fixed in #4223.