Assign GitHub Topics and URL for all Contrib Projects

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GitHub repositories have the ability to add "Topics" and a project URL.

@yorkshire-pudding pointed out that this can help discoverability of contrib modules, and also increase the cross-linked traffic to BackdropCMS.org.

There is an API for managing topics, so we could automate the process of adding topics to all Backdrop contrib repositories.

@yorkshire-pudding recommends the following topics minimum:

• backdrop
• backdropcms
• cms

And we should set the project URL to https://backdropcms.org/project/[project_name]

Because BackdropCMS.org receives webhooks for most GitHub contrib module activities, we could leverage this existing connection to automatically add topics and URLs to contrib modules.

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This was part of Backdrop LIVE April 4, 2024: https://events.backdropcms.org/discussions/how-your-contrib-repo-can-help-to-grow-backdrop

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We could copy some of the project_github_githubapi_payload() code, where the README.md gets copied onto BackdropCMS.org. It could be made part of project_github module but it would be a lot simpler to make it part of our custom modules.