Setup slack notifications on explorer pipelines

Following what has been done on the platform with NXP-29338, add slack notifications on:

• explorer master branch build (on failure, and on success only when build was previously failing - except if UNSTABLE as ftests show regularly randoms on ES indexing worker)
• explorer release (when not in dry run, success and failure)
• explorer export (only when uploading, on failure and on success only when build was previously failing)
• triggers of explorer export (only on failure)
• cleanup script (only on failure)

Notifications can go to #platform-notifs Slack channel for now.