Fixes #3551

This PR introduces a GitHub Action that allows contributors to assign or unassign themselves from issues and pull requests using simple comment commands.

Changes:

• /assign: Assigns the commenter to the issue or PR.
• /unassign: Removes the commenter as an assignee.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123