Improve navbar code and support names which are different from links

Right now the links on the navbar are stored in _config.yml. I think they should be put in their own file, for example _data/menu.yaml, so that config.yml only contains stuff that needs to be there. In addition, support should be added for link names which are separate from the URLs themselves, as currently the link names are created by taking the URL and capitalising the first character.

This would add support for link names with spaces in them and for links which are abbreviated from the link names, making the navbar more versatile. As a result, it would also be more intuitive to make edits to the navbar.