Add editable templates for bulk email

Adds the edX Marketing-approved template as html default.

lms/djangoapps/bulk_email/forms.py

@@ -0,0 +1,42 @@
+import logging
+
+from django import forms
+from django.core.exceptions import ValidationError
+
+from bulk_email.models import CourseEmailTemplate, COURSE_EMAIL_MESSAGE_BODY_TAG
+
+log = logging.getLogger(__name__)
+
+
+class CourseEmailTemplateForm(forms.ModelForm):
+    """Form providing validation of CourseEmail templates."""
+
+    class Meta:
+        model = CourseEmailTemplate
+
+    def _validate_template(self, template):
+        """Check the template for required tags."""
+        index = template.find(COURSE_EMAIL_MESSAGE_BODY_TAG)
+        if index < 0:
+            msg = 'Missing tag: "{}"'.format(COURSE_EMAIL_MESSAGE_BODY_TAG)
+            log.warning(msg)
+            raise ValidationError(msg)
+        if template.find(COURSE_EMAIL_MESSAGE_BODY_TAG, index + 1) >= 0:
+            msg = 'Multiple instances of tag: "{}"'.format(COURSE_EMAIL_MESSAGE_BODY_TAG)
+            log.warning(msg)
+            raise ValidationError(msg)
+        # TODO: add more validation here, including the set of known tags
+        # for which values will be supplied.  (Email will fail if the template
+        # uses tags for which values are not supplied.)
+
+    def clean_html_template(self):
+        """Validate the HTML template."""
+        template = self.cleaned_data["html_template"]
+        self._validate_template(template)
+        return template
+
+    def clean_plain_template(self):
+        """Validate the plaintext template."""
+        template = self.cleaned_data["plain_template"]
+        self._validate_template(template)
+        return template