allow forking a renamed version of a notebook

[gordonwoodhull]

Although renaming the notebook might or might not make sense when it's on a historical version, Forking & Renaming with ctrl/cmd enter definitely makes sense.

It's what you do when you realize you should have forked before you started changing something.

[gordonwoodhull]

It's a little weird, but you can do this in the history in the notebook tree - just start to rename the version as if you are creating a tag, but press ctrl/cmd enter instead. I don't know if this is more or less confusing than if we enabled it in the navbar, which would also mean being able to change the title of all versions while viewing an old version.