New Workflow for managing stale issues and PRs

[wahln]

Adds a "stale" workflow to mange stale issues and PRs. Needs to be merged directly into the master branch.

• issue marked as stale after 14 days of no activity with comment & label. Exemptions for some labels.
• stale issue closed after 14 more days of no activity
• PRs marked as stale after 30 days of no activity with comment & label
• PRs will not be automatically closed
• Job will run each day at midnight