Modernize JS a little

[yuvipanda]

• Get rid of var
• Use class syntax rather than prototype syntax for classes
• Remove unused param
• Split classes to their own files

[yuvipanda]

Based on #272

[yuvipanda]

See https://github.com/jupyterhub/nbgitpuller/pull/273/commits/590b3fc1a8687bd93a30ec974d70f8a742f91797 for just the changes in this PR

[yuvipanda]

Thanks @GeorgianaElena