Add milestone syncing

[mgwalker]

Description of changes

Adds an automation to sync creation of milestones. When a milestone is created in this repo, the same milestone is also created in the ux-guide repo.

• Addresses 18F/guides#297

  Known issues

• does not update milestones if they are changed

Related issues

• closes 18F/guides#297

Reviewers:

• [x] @cannandev: Please review the GitHub Actions code. It is identical to 18F/methods#653
• [x] @MelissaBraxton and/or @bpdesigns: Please review the behavior describe above to make sure it's right.

[bpdesigns]

@mgwalker as a follow on is it possible to do what you did with labels? that Automation resulted in whenever an update happens in the labels in one repo its translated to the other repo as well. The outcome I'm looking for is if a milestone is created in either repo it syncs the milestone to the other repo.

[mgwalker]

@bpdesigns Yes! 18F/methods#653 adds this to the methods repo as well. The two repos will "push" to each other, so once they're both merged, creating a milestone in either repo will cause the same milestone to be created in the other repo.