Added Bootstrap Nav Walker + Navigation Header to the default theme.

[mavieth]

I added a default bootstrap nav walker to the theme template. I have found that each time I use the Sage theme, I end up using the default Bootstrap navigation, and figured this would be nice for people who do not know how to implement it at first.

[AdamWills]

This has previously been taken out of the project as there's a push to make Sage more framework agnostic.

[paultibbetts]

It was removed in #1427

Somebody asked about it in #1475 and someone else re-added it in #1649

There's nothing stopping you from keeping it in your own fork of Sage though 😃