Refactored content link generation

Content link generation separated into multiple functions for clarity
Bolt\Content::link no longer throws an exception for unroutable content
Updated backend & sitemap templates for unroutable content

I have not worried too much about the themes, as my general thought process is that if you have unroutable content, then you are most likely working with a custom theme to deal with this anyways.

If you look at the function I have called `getRouteRequirementParams`, it becomes clear that the only field/taxonomy/relation values which can be used as route parameters are those which have explicitly defined requirements in `routing.yml`. Is this the correct behavior, or should values for 'simple' field types be added as well? How about slugs for taxonomies/relations?