PR view says a pull request in a private repository doesn't exist #160

Description:

For a pull request in a private repository, /github <owner>/<repo> pulls <n> and the View Changes button in search results open the PR view with "Pull request # doesn't exist.", even when the user has logged in with /github login. View Changes and View File inside that view fail for private repositories too.

Steps to reproduce:

1. Run /github login and finish the GitHub sign-in.
2. Run /github <owner>/<private-repo> pulls <n> for a pull request that exists.

Expected behavior:

The PR view opens with the title, author, state and changed files, and View Changes and View File show their content.

Actual behavior:

The view says "Pull request # doesn't exist."

Rocket.Chat Server Setup Information:

• Version of Rocket.Chat Server: 8.8
• Operating System: Linux
• Deployment Method: docker

Rocket.Chat Client Setup Information

• Browser Version: Chrome, web client

Additional context

modals/pullDetailsModal.ts fetches api.github.com/repos/{repo}/pulls/{n} and /files without an Authorization header, so GitHub answers 404 for a private repository. modals/fileCodeModal.ts fetches the diff_url and raw_url links without a token as well. Private repository support was added for repo details in #21 (#19) but not for this view. The message comes from #141 (#140).