Describe the pull request

When the wiki for a repo is enabled, but doesn't have content yet, pushing to the wiki's git url will always fail.
This change provides a configuration option to automatically create an empty git repo, which can be pushed to.

Link to the issue: #7226

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code or have provided the test plan.

Test plan

1. Enable repository.AUTOCREATE_WIKI in app.ini
2. create a new repo eg owner/repo and enable the wiki for that project
   • the wiki should now be empty and ask to create the first new page
   • no repo.wiki.git is yet available on the server
3. push to git@gogs:owner/repo.wiki.git
   • the repo.wiki.git is initialized
   • the content of the push is stored on the server
   • If there are Markdown-files in the wiki, the wiki-side displays them