Handle private GitHub repos on Config Page

[AlexTugarev]

Fixes #6168

To be more specific what this PR resolves and how to verify:

1. ensure you have just user:email permissions for GitHub on https://at-config-private-repo-6168.staging.gitpod-dev.com/integrations
2. go to https://at-config-private-repo-6168.staging.gitpod-dev.com/new and import a private repo from your account as a project
3. on the configuration page you should now see the Authorize button
4. do authorize and grant repo permissions
5. the project configuration fetch should be repeated automatically

Handle private GitHub repos on Config Page

[jldec]

@AlexTugarev I tried this on https://at-config-private-repo-6168.staging.gitpod-dev.com/
I installed my personal github app there to test the flow with a private github repo.

I noticed 2 things:

1. this PR is based on a fairly old commit on main - it would be good to rebase/merge the recent commits e.g. to include the changes to the "New Workspace" button.

2. I'm seeing the new no-access notice in the pane on the left 👍 , but I still get unhandled errors in the console, both before and after clicking the "Authorize Provider" button, and then the page is stuck with no way to continue.

Uncaught (in promise) Error: Request guessProjectConfiguration failed with message: NotFoundError
Uncaught (in promise) Error: Request fetchProjectRepositoryConfiguration failed with message: NotFoundError

[jldec]

This works well for me now, thank you @AlexTugarev !

[roboquat]

LGTM label has been added.

Git tree hash: 83c1388b6df44c338b1b3b272eeaf978e209346b

[roboquat]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jldec

Associated issue: #6168

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