[server] Fix the 'addComment' and 'addBadge' prebuild features in the Gitpod GitHub App

[jankeromnes]

This PR fixes Gitpod's addComment and addBadge prebuild features by updating our GitHub App code to the latest Probot/Octokit API.

It also fixes:

ERROR    (event): Cannot read property 'update' of undefined
    TypeError: Cannot read property 'update' of undefined
        at GithubApp.onPrAddBadge (/app/node_modules/@gitpod/server/dist/ee/src/prebuilds/github-app.js:241:57)
        at GithubApp.<anonymous> (/app/node_modules/@gitpod/server/dist/ee/src/prebuilds/github-app.js:171:22)
        at Generator.next (<anonymous>)
        at fulfilled (/app/node_modules/@gitpod/server/dist/ee/src/prebuilds/github-app.js:22:58)
        at runMicrotasks (<anonymous>)
        at processTicksAndRejections (internal/process/task_queues.js:97:5)

and:

WARN     (event): [probot] "context.github" is deprecated. Use "context.octokit" instead.

How to test:

• Open PRs against any repository configured with the gitpod-staging GitHub App, for example https://github.com/jankeromnes/gitpod-staging-prebuilds
• The PR should receive a "badge" (button in the PR comment), a comment with a button, and a PR check. All buttons should work, have a non-broken image, and open the PR in Gitpod
• The server shouldn't log errors related to GithubApp

[jankeromnes]

Hot-patched https://gitpod-staging.com with this PR's server image.

• Old button URLs are still being redirected: https://gitpod-staging.com/api/apps/github/pbs/github.com/gitpod-io/gitpod/5431d5735c32ab7d5d840a4d1a7d7c688d1f0ce9.svg

• And now PR comments are fixed: Create README.md jankeromnes/gitpod-staging-prebuilds#5 (comment) 🎉

[jankeromnes]

• And now addBadge also works again: Create README.md jankeromnes/gitpod-staging-prebuilds#6 (comment)

[akosyakov]

It works as advertised.