features: Update screenshot and add tool to generate.

This adds a new management command that should make generating or
updating screenshots a bit easier in the future.

tools/linter_lib/custom_check.py

@@ -267,6 +267,7 @@ def build_custom_checkers(by_lang):
         {'pattern': '[^r][M]essage.objects.get',
          'exclude': set(["zerver/tests",
                          "zerver/lib/onboarding.py",
+                         "zilencer/management/commands/add_mock_conversation.py",
                          "zerver/worker/queue_processors.py"]),
          'description': 'Please use access_message() to fetch Message objects',
          },