Add size limit for uploading user avatars and realm icons.

- Add settings parameter for max realm icon size.
- Add settings parameter for max user avatar size.
- Add checking file size to avatar and icon
  uploading views.
- Transfer file size limit parameter to frontend.
- Add tests.

zproject/settings.py

@@ -116,6 +116,8 @@ DEFAULT_SETTINGS = {'TWITTER_CONSUMER_KEY': '',
                     'S3_AVATAR_BUCKET': '',
                     'LOCAL_UPLOADS_DIR': None,
                     'MAX_FILE_UPLOAD_SIZE': 25,
+                    'MAX_AVATAR_FILE_SIZE': 5,
+                    'MAX_ICON_FILE_SIZE': 5,
                     'ERROR_REPORTING': True,
                     'BROWSER_ERROR_REPORTING': False,
                     'STAGING_ERROR_NOTIFICATIONS': False,