feat: add govDelivery listing opportunity email

[ludtkemorgan]

This PR addresses #6220

• Addresses the issue in full
• Addresses only certain aspects of the issue

Description

Doorway has the same listing opportunity email that is in core except it goes through govDelivery rather than directly through the email service.

This PR does the following:

• Adds a govDelivery service to be able to send bulletins to defined lists
• sends the listing opportunity email via govDelivery if enableListingOpportunity feature flag is enabled
• Updates the listing opportunity email to have features only seen in Doorway:
  • Disclaimer text below the apply buttons
  • Additional footer text
  • Bolded the due date field - This has been done everywhere

Open questions:

1. The time has been added to the due date field ("May 19, 2026 at 3:00pm PDT" instead of "May 19, 2026"). I added it for all cases, but need to get verification that that can be done for LA.

How Can This Be Tested/Reviewed?

This is hard to test without access to govDelivery. You can work with Morgan to get added to the staging govDelivery list

The "New Rental Opportunity" email with correct content should be sent in the following scenarios:

1. when a listing is created and the status is "active"
2. when a listing is updated and the status is "active" when it previously wasn't ("publish" the listing)

Author Checklist:

• Added QA notes to the issue with applicable URLs
• Reviewed in a desktop view
• Reviewed in a mobile view
• Reviewed considering accessibility
• Added tests covering the changes
• Made corresponding changes to the documentation
• Ran yarn generate:client and/or created a migration when required

Review Process:

• Read and understand the issue
• Ensure the author has added QA notes
• Review the code itself from a style point of view
• Pull the changes down locally and test that the acceptance criteria is met
• Either (1) explicitly ask a clarifying question, (2) request changes, or (3) approve the PR, even if there are very small remaining changes, if you don't need to re-review after the updates

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