Update supplementary github files to improve repo organization and maintenance

[dlstadther]

Description

• Adds .github/ directory for organization
• Adds configuration for Stale Probot

Motivation and Context

The luigi repo has been getting cluttered overtime from abandoned feature requests, bug reports, help requested, and code submissions. The goal of adding Stale is not to keep ignoring contributors' interactions, but rather to keep those of recent relevance and active addition in focus.

There are two sides to make this an effective solution to help improve the quality (both actual and perceived) of spotify/luigi.

1. contributors need to actively follow their PRs/Issues from submission to merge/close (myself included)
   • it's ok if you don't have time to continue working on them - it's fine to close and return to them at a later date
   • perhaps we need to create a label for those items which will be completed, just at a later date
2. maintainers need to provide feedback at quicker intervals to allow for less stale PRs/Issues (again, myself included)
   • perhaps we add a .github/CODEOWNERS to auto tag certain maintainers or other frequently active contributors to aid in the code review speed up process
   • if others are in favor, i'd be happy to add this file with primary maintainers to start

Have you tested this? If so, how?

No testing needed - supplementary github files

[Tarrasch]

Awesome!