Fix @here in holiday reminders

[mgwalker]

Fixes an issue where @here was showing up as just @. Slack previously supported a mechanism for changing the @here text while preserving the notification. Something changed somewhen, and specifying that alias now seems to result in just the @. So... remove the alias, and we're back in business.

Closes #511

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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