Refactor and ES6-ify `PDFLinkService.navigateTo`
Refactor and ES6-ify `PDFLinkService.navigateTo`

This patch replaces a `var self = this;` statement with arrow functions, and `var` with `let` in `PDFLinkService.navigateTo`.

Furthermore, when I started looking at this method, it quickly became clear that its code is somewhat of a mess. Since I'm one of the persons that have touched this code over the years, I figured that it'd be a good idea to try and clean it up a bit.

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