Make the branch creation button check simpler

What does this MR do?

It changes the behaviour of the "Create new branch" button, so that it shows when there is a related branch, and just hides the complete button if the issue has been closed

Why was this MR needed?

I was responding to an issue that I closed earlier today, and then I saw that the button says: "New branch is unavailable". And my first assumption at that point was, that the button was broken. So the behaviour felt unexpected to me. By changing it around a little bit I've tried to make the behaviour more "logical"

Are there points in the code the reviewer needs to double check?

@zj It would be great if you can take a look as well, since you are the original author of the code.

Open todos

  • Create tests (Will do after the first review pass is done, and I know this new behaviour has the change to be merged.)