Disable plugin download and install if the user has not provided a Personal Access Token (PAT) to GitHub

[TheWitness]

Feature Request

Is your feature request related to a problem? Please describe

1. We need to warn the user upon install that a PAT is preferred and how to get one at install time.
2. We need to provide a message on the plugins page if one is not set.
3. We need to block the automatic download if the PAT has not been provided.

Describe alternatives you've considered

Letting it silently fail due to the Rate Limiting in GitHub.

Additional context

We agreed to this at our monthly developers meeting.