Move suggested_prices to CharField for Django 1.11 compat

CommaSeparatedIntegerField is deprecated, this is the replacement. I
have confirmed there is no SQL generated for this migration since it is
already a CharField behind the scenes.

common/djangoapps/course_modes/migrations/0009_suggested_prices_to_charfield.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import re
+import django.core.validators
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('course_modes', '0008_course_key_field_to_foreign_key'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='coursemode',
+            name='suggested_prices',
+            field=models.CharField(default=b'', max_length=255, blank=True, validators=[django.core.validators.RegexValidator(re.compile('^[\\d,]+\\Z'), 'Enter only digits separated by commas.', 'invalid')]),
+        ),
+    ]

common/djangoapps/course_modes/models.py

@@ -8,11 +8,11 @@ import pytz
 from config_models.models import ConfigurationModel
 from django.conf import settings
 from django.core.exceptions import ValidationError
+from django.core.validators import validate_comma_separated_integer_list
 from django.db import models
 from django.db.models import Q
 from django.dispatch import receiver
 from django.utils.translation import ugettext_lazy as _
-from django.utils.encoding import force_text
 from openedx.core.djangoapps.content.course_overviews.models import CourseOverview
 
 from opaque_keys.edx.keys import CourseKey
@@ -103,7 +103,8 @@ class CourseMode(models.Model):
     # DEPRECATED: the suggested prices for this mode
     # We used to allow users to choose from a set of prices, but we now allow only
     # a single price.  This field has been deprecated by `min_price`
-    suggested_prices = models.CommaSeparatedIntegerField(max_length=255, blank=True, default='')
+    suggested_prices = models.CharField(max_length=255, blank=True, default='',
+                                        validators=[validate_comma_separated_integer_list])
 
     # optional description override
     # WARNING: will not be localized