On Gitea instance, allow pull requests on any fork

[andy5995]

@Stan` @ItmsI suggest allowing pull requests on any fork. For example, I was going to help with https://gitea.wildfiregames.com/0ad/0ad/pulls/7075

So I pulled the branch and was going to open a pull request on abian's fork:with-lto branch. On GitHub, when this happens, the user can merge the pull request on his fork. If he does so, the already-open PR (link above) on the main upstream repo gets updated.

You can see here i don't have the option to open a PR on his fork. It only allows me to open a pull request on my fork or the 0ad repo:

 

[Itms]

That is something everyone can activate, @abian can activate PRs on their fork. I do not think there is an option to activate that by default and I'm not sure it's desirable (in terms of UI clutter).