Add label activity event.

[BURG3R5]

Labels are helpful tags which users can use to sort and manage issues/PRs for their GitHub repository. From the documentation:

You can manage your work on GitHub by creating labels to categorize issues and pull requests. You can apply labels in the repository the label was created in. Once a label exists, you can use the label on any issue or pull request within that repository.

This issue consists of adding a new event for the bot to track. You'll be updating the bot to make sure that users can be notified when labels are added/modified/deleted in their repository.

These are the steps to do so:

• Add LABEL_UPDATED to EventType enum. #7
• Add new Label model. #8
• Add new GitHub parser called LabelUpdateEventParser. #9
• Add new elif clause to compose_message for LABEL_UPDATED event. #10

More information can be found in the individual issues.
Leave a comment if you have any doubts.