Fork internal PRs

As posted on the gin.g-node.org Info Issue list.

**Issue**

The 'New Pull Request' button behaves differently depending on whether it is clicked from the 'Pull Requests' website or from the website of a specific PR when operating on a fork.
**Situation**

I have made a fork of an upstream repository, let's call that one 'fork'. I created a new branch 'fork/new_branch' with some changes. Now I want to create a PR onto fork/master. However, when clicking on the 'Pull Requests' tab and then clicking on 'New Pull Request', I am unable to choose the target location, and by default, 'upstream/master' is selected. However, when I click on an existing PR in that fork, and then click on 'New Pull Request', the target location is set to fork/master.

Ideally, both buttons would behave the same and also let me choose whether I want to target upstream/master or fork/master.