Simplify site nav generation

[ang-zeyu]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [ ] Documentation update
• [ ] Bug fix
• [ ] New feature
• [ ] Enhancement to an existing feature
• [x] Other, please explain:

Code maintainability

What is the rationale for this request?

• simplify the recursive algorithm used in site nav generation to an iterative one
• should also bring some performance benefits since we are no longer constantly reparsing / outputting the html across recursive calls of formatSiteNav

What changes did you make? (Give an overview)

• merged formatSiteNav into insertSiteNav since the recursion is removed
• the new algorithm simply iterates through ul elements one by one, and its direct li child elements, formatting it the same way the old algorithm did
• extract some more constants to constants.js

Testing instructions:

• npm run test should pass ( no test file updates here )

Proposed commit message: (wrap lines at 72 characters)
Simplify site nav generation

The site nav generation algorithm is heavily recursive.

Let's remove the recursion in favour of a simple iterative process.

[ang-zeyu]

handled by navigationElements.length === 0 below