Refactoring html_shell.rs to improve readability

[Asha20]

Hi, I was wondering if I could contribute to the project by helping improve general code quality?

I picked this file in particular as a starting point because it seemed rather small and self-contained, but if you have other suggestions please feel free to let me know.

Here are some of the things I feel could be improved:

• Remove code duplication here and here.
• Use newtypes to enforce invariants present in comments here and here.
• Try to improve readability of the numerous format! calls that embed other strings inside HTML fragments, probably by putting them in separate functions.

[arctic-hen7]

You're more than welcome to contribute, please go right ahead and open a PR! A very large portion of Perseus is fairly haphazardly written because of how many times I've needed to change things to add new features. I18n in particular turned things into a bit of a pigsty. Any refactoring is appreciated, thank you very much!

I will be doing a major refactor soon to close #4, but that's a little while away.