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/open.go

@@ -0,0 +1,33 @@
+package classes
+
+import (
+	"strings"
+	"unicode/utf8"
+
+	"git.golem15.com/golem15/sm-quizzes-plugin/models"
+)
+
+// MaxLength is the longest answer an open question accepts, in characters:
+// its MaxLength when set and positive, else models.DefaultMaxLength.
+func MaxLength(q models.Question) int {
+	if q.MaxLength != nil && *q.MaxLength > 0 {
+		return *q.MaxLength
+	}
+	return models.DefaultMaxLength
+}
+
+// CheckOpenText cleans a student's answer to an open question (D-04): it
+// removes every NUL character (Postgres text refuses them) and the
+// surrounding whitespace. A blank text is a FieldError text_required and a
+// text longer than MaxLength(q) characters (runes, not bytes) is a
+// FieldError text_too_long.
+func CheckOpenText(text string, q models.Question) (string, error) {
+	text = strings.TrimSpace(strings.ReplaceAll(text, "\x00", ""))
+	if text == "" {
+		return "", &FieldError{Field: "text", Key: "text_required"}
+	}
+	if utf8.RuneCountInString(text) > MaxLength(q) {
+		return "", &FieldError{Field: "text", Key: "text_too_long"}
+	}
+	return text, nil
+}