Branch model should not allow branch category overlap

We've had multiple support requests now where customers have set their "develop" branch to something with a release/ prefix, meaning it is also a "release" branch. This causes auto-merge to misbehave.

Some options might be:

• Throw an exception when customers try to save a branch model where the categories overlap (Should be pretty easy to detect on the server)
• When the "develop" branch appears in the "release" chain, remove it and always keep it at the end
• When the "develop" branch appears in the "release" chain, leave it and drop it from the end

There are probably other options as well; those are just off the top.