[BUGFIX] Fix resource-bundle links to point to the correct location

Problem/Motivation

As https://www.drupal.org/node/2751985#comment-11331069 suggests, we want to change the related link in the resource-bundle relationship to point to /node/article/1 instead of to /node/1/resource-bundle.

Proposed resolution

Extend the Relationship class to create a BundleRelationship class that overrides the link generation methods. In that overridden method we can point to the correct URL for that relationship.

Remaining tasks

This is blocked until #2755025: [CLEANUP] Refactor relationships to have a unified base class and #2751985: [FEATURE] Add a bundle level relationship to entity level resources are merged. There is also some discussion pending to wether this is a good idea or not. See https://www.drupal.org/node/2751985#comment-11330117.