Enable GitHub integration to fetch information from public repositories

Currently the GitHub integration seems to only fetch information from the user/organization that created the application for the integration.
So if other GithHub links are mentioned in a document, the hover-over does not show any information. The url.unfurl request for public repos also returns a 204, while the one for a repo in the connected organization returns a 200.

<img width="406" height="113" alt="image" src=" />

Im not sure this is the intended behavior, but the flavor text when configuring the GitHub integration suggests this is intended.

> Enable previews of GitHub issues and pull requests in documents by connecting a GitHub organization or specific repositories to Outline.

It would be nice, if information on public issues and PRs were also displayed in the hover-over.