Do not list forks

[xeronimus]

"Contributions" should not list forks of other open source repositories where Zühlke is only a contributor.

Details

• contributions.json should only contain "our own repos"
• when collecting organization repos (github_api.py -> collect_org_repos) filter out forks
• github API response contains flag fork

e.g.

{
    "id": 152042863,
    "node_id": "MDEwOlJlcG9zaXRvcnkxNTIwNDI4NjM=",
    "name": "i18n",
    "full_name": "Zuehlke/i18n",
    "private": false,
    "owner": {
      "login": "Zuehlke",
      "id": 10219568,
    },
    "html_url": "https://github.com/Zuehlke/i18n",
    "description": "A plugin that provides i18n support.",
    "fork": true
    ...