Make card title mandatory

We didn't add any validation rules to the card model. At the very least,
the title should be mandatory.

The fact that the label field is marked as optional in the form but the
other fields are not probably means description and link should be
mandatory as well. However, since there might be institutions using
cards with descriptions but no link or cards with links but no
description, so we're keeping these fields optional for compatibility
reasons. We might change our minds in the future, though.

spec/models/widget/card_spec.rb

@@ -9,6 +9,10 @@ describe Widget::Card do
     it "is valid" do
       expect(card).to be_valid
     end
+
+    it "is not valid without a title" do
+      expect(build(:widget_card, title: "")).not_to be_valid
+    end
   end
 
   describe "#header" do