Removing duplicate code and keeping File API consistent

- Removing repeated code for `Link` and `URL`
- Adding comments to `Link` functions
- Joining links correctly

Personally, I'd be in favour of deprecating `getURL` and `getAbsoluteURL` in favour of keeping a consistent API with `SiteTree`, but this is a solution to keep both happy without redundantly implementing the same functionality but with slight differences
Ideally; though we do want to have consistency across the fairly important module :P

This function is just an alias of the other function which should be described as above. I can put that here and there, but that's not very DRY