Configure an API key for stale issues reminder

Ref: #53 (comment 153928)

• Decide what level of authorization the user must have.
• Create a GitLab user for the stale issues reminder
• Generate an API key
• Add the API key to the place where it'll be used (ticket triager machine, puppet-tails_secrets_jenkins, puppet-lizard-hiera-node, or what not, depending on where the issue reminder code will run