Add avatar_version to UserProfile.

avatar_version is used to keep track of the avatar version of the
user and to update the cache, everytime the user updates their avatar.

zerver/migrations/0050_userprofile_avatar_version.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.5 on 2017-01-23 17:44
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('zerver', '0049_userprofile_pm_content_in_desktop_notifications'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='userprofile',
+            name='avatar_version',
+            field=models.PositiveSmallIntegerField(default=1),
+        ),
+    ]

zerver/models.py

@@ -546,6 +546,7 @@ class UserProfile(ModelReprMixin, AbstractBaseUser, PermissionsMixin):
             (AVATAR_FROM_USER, 'Uploaded by user'),
     )
     avatar_source = models.CharField(default=AVATAR_FROM_GRAVATAR, choices=AVATAR_SOURCES, max_length=1) # type: Text
+    avatar_version = models.PositiveSmallIntegerField(default=1) # type: int
 
     TUTORIAL_WAITING  = u'W'
     TUTORIAL_STARTED  = u'S'