feat: add open questions with a pending grading state

- migration: open question type, rubric, max_length, quiz grader and
  the grading and review columns of attempt answers
- open answers are checked, stored as pending and never scored on
  answer; a changed text resets the grading state
- student JSON gains text, grading_status, grader_feedback,
  review_comment, max_length, needs_review and open_answers; the
  rubric is never serialized

classes/scoring_test.go

@@ -1,6 +1,7 @@
 package classes_test
 
 import (
+	"math"
 	"os"
 	"path/filepath"
 	"testing"
@@ -130,3 +131,37 @@ func TestScoringConfigFrom(t *testing.T) {
 		}
 	}
 }
+
+func TestClampOpenPoints(t *testing.T) {
+	cases := []struct {
+		p      float64
+		points int
+		want   float64
+		ok     bool
+	}{
+		{1.26, 2, 1.5, true},
+		{-3, 2, 0, true},
+		{7, 2, 2, true},
+		{0.74, 3, 0.5, true},
+		{0.25, 3, 0.5, true},
+		{0.2, 3, 0, true},
+		{2, 2, 2, true},
+		{1, 0, 0, true},
+		{5, -1, 0, true},
+		{math.NaN(), 2, 0, false},
+		{math.Inf(1), 2, 0, false},
+		{math.Inf(-1), 2, 0, false},
+	}
+	for _, tc := range cases {
+		got, ok := classes.ClampOpenPoints(tc.p, tc.points)
+		if got != tc.want || ok != tc.ok {
+			t.Errorf("ClampOpenPoints(%v, %d) = %v, %v; want %v, %v", tc.p, tc.points, got, ok, tc.want, tc.ok)
+		}
+		if math.Signbit(got) {
+			t.Errorf("ClampOpenPoints(%v, %d) is negative zero", tc.p, tc.points)
+		}
+	}
+	if !classes.OpenCorrect(2, 2) || classes.OpenCorrect(1.5, 2) || classes.OpenCorrect(0, 0) {
+		t.Error("OpenCorrect: want true only for full points of a question worth points")
+	}
+}