DevOps: automate PR management tasks

[nlebovits]

Describe the task

Set up GitHub Actions to automate PR management tasks. Specifically, we need to:

1. Automatically label and assign reviewers to PRs.
2. Flag (but not close) PRs that are more than a week old.

Acceptance Criteria

• A GitHub Action is created that automatically assigns labels and reviewers when a PR is opened.
• A GitHub Action is created that flags PRs that are older than 7 days with a "stale" label.
• Both actions are tested and confirmed to be working as expected.

Additional context

Back end PRs pertaining to the data/ subdirectory should be reviewed by @zigouras, while front end PRs pertaining to the root directory and src/ should be reviewed by @CodeWritingCow.

[nlebovits]

closed by #795