chore: Refactor build task queue job processor

[apburnes]

Closes https://github.com/cloud-gov/pages-core/issues/4496

Changes proposed in this pull request:

• Refactors the build task job processor to pull all build info in the buildTaskRunner job process
• Refactors the Mailer queue to follow the conventions of all other queues
• Moves Mailer queue jobs into the QueueJobs class
• Removes unused Slack queue and jobs

security considerations

None