OpportunityList.explanation field is not allowed to be blank

[Dylan McCall]

If I open /admin and start editing an opportunity list, I can't save changes unless I set the Explanatory instructions field to something. That is not the correct behaviour; this should be an optional field. In addition, the field has null=True, which is not recommended for text fields.

Meanwhile, the experience field has two blank choices here. One generated by Django (with the label "-------"), and another by us (with a blank label). Removing blank=True fixes that, removing Django's own blank choice (which seems to map to an empty string, leading to misery and destruction). Unfortunately that also makes the admin interface show “Required Experience” as if it needs to be set to something, unlike a BooleanField for example, but I think we can deal with that.