Ensure only defined annotations are allowed

[robrap]

At this time, users are not given any linting warnings when undefined annotations are used, including misspellings.

Additional Notes:

• An example: someone might use toggle_warnings instead of toggle_warning.
• It has been noted that removing the ability to comment between annotations (see Remove ability to comment between annotations #89) would make this simpler to implement.
• Linting might be implemented here: https://github.com/openedx/edx-lint/blob/7573005f89df7f78142a2af86673206e8b43c4d5/edx_lint/pylint/annotations_check.py#L291