settings: Add support for uploading logo for night mode.

This adds a new field named realm_night_logo which is used for
displaying the organization logo when the user is in night mode.

Fixes #11176.

zerver/migrations/0208_add_realm_night_logo_fields.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.18 on 2019-01-15 16:07
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0207_multiuseinvite_invited_as'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='realm',
+            name='night_logo_source',
+            field=models.CharField(choices=[('D', 'Default to Zulip'), ('U', 'Uploaded by administrator')], default='D', max_length=1),
+        ),
+        migrations.AddField(
+            model_name='realm',
+            name='night_logo_version',
+            field=models.PositiveSmallIntegerField(default=1),
+        ),
+    ]