allow a Repository to be created or configured with PullRequest settings

[jstrachan]

The UI lets you configure a Repository to Allow Pull Request merge commits and so forth:

when creating a Repository via the go API all these are false. This means if you create a Pull Request there's no UI in gitea that lets you merge the PR.

I don't see any way to specify or default these values in the go-gitea REST API - so it'd be nice to add these flags so we can pass them into the CreateRepo method

[jstrachan]

here's how a PR looks when you raise it: