Ensure onion_address setting includes http:// protocol

The Onion Location Middleware requires this setting to act as a "base"
URL in order to form the correct header value, so we need the
`http://` in this setting.  The validation already tries to make sure
this happens, so we ought to have it in the default value as well.

common/migrations/0006_auto_20201005_1953.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.2.14 on 2020-10-05 19:53
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('common', '0005_auto_20200810_2049'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='footersettings',
+            name='securedrop_onion_address',
+            field=models.CharField(default='http://secrdrop5wyphb5x.onion', help_text='Address for the securedrop.org onion service. Displayed in the site footer and the Onion-Location header.  Must begin with http or https and end with .onion', max_length=255, validators=[django.core.validators.RegexValidator(message='Enter a valid .onion address.', regex='\\.onion$'), django.core.validators.URLValidator(message='Onion address must be a valid URL beginning with http or https', schemes=['http', 'https'])], verbose_name='SecureDrop Onion Address'),
+        ),
+    ]

common/models/settings.py

@@ -43,7 +43,7 @@ class FooterSettings(BaseSetting):
     securedrop_onion_address = models.CharField(
         'SecureDrop Onion Address',
         max_length=255,
-        default='secrdrop5wyphb5x.onion',
+        default='http://secrdrop5wyphb5x.onion',
         validators=[
             RegexValidator(regex=r'\.onion$', message="Enter a valid .onion address."),
             URLValidator(schemes=['http', 'https'], message='Onion address must be a valid URL beginning with http or https'),