Filter Add / Remove activities with non-Note payloads (resolves #750)

What does this PR do?

Ignore Add / Remove activities where the payload is not a Note. This prevents an exception when a Mastodon user updates their pinned tags.

Contribution Guidelines By submitting this merge request, you agree to follow our Contribution Guidelines

• I agree to follow this project's Contribution Guidelines
• I have made sure to test this pull request