tracking-issue: Implement simplified workflow

Created by: tsenart

This PR changes the tracking-issue tool to simplify how we create tracking issues and have them be automatically updated.

Before, changing which tracking issues are automatically updated required a change and pull request to the "Tracking Issue Syncer" GitHub Action definition. This needed to happen on every milestone change, for instance.

With this change, the tool instead finds all open tracking issues in the sourcegraph repo that have the tracking label, searches for all their issues and PRs (that have the rest of the labels in the tracking issue), and updates them automatically when there are modifications.

Additionally, this change set also makes it possible for tracking issues to be created for any other label, even if the tracking issue isn't bound to a milestone. This allows us to have tracking issues for things like RFCs, OKRs, etc. cc @nicksnyder

All of this will be documented properly in https://github.com/sourcegraph/about/pull/786.