Introduce the concept of "optional" triggers in the Inclusion Bot wordlist and support opting out of those notifications

[mgwalker]

This PR proposes to introduce a tiered structure to the trigger phrases in the Inclusion Bot list. The first tier would be those terms that we agree should be flagged in all circumstances. The second tier would be those terms that we think people should be aware of but they may not want to be reminded of regularly.

This PR allows us to designate a term as optional and allows users to opt-out of "optional" notifications. If a Slack user says one of the optional phrases and is opted out, they will not get a notification about it.

I'm hopeful that this furthers Inclusion Bot's mission of education and thoughtfulness in a way that allows people some agency if they decide that they do not wish to change their behavior around some terms, while still allowing us to constantly remind people about language that we believe is more universally uninclusive.

This update does NOT make any changes to the word list. The entire word list is still consider mandatory. Making individual trigger words/phrases "optional" would be a separate task.

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Checklist:

• Code has been formatted with prettier
• The OAuth wiki
  page has been updated if Charlie needs any new OAuth events or scopes
• The Environment Variables
  wiki page has been updated if new environment variables were introduced
  or existing ones changed
• The dev wiki has been updated, e.g.:
  • local development processes have changed
  • major development workflows have changed
  • internal utilities or APIs have changed
  • testing or deployment processes have changed
• If appropriate, the NIST 800-218 documentation has been updated