SS5: Remove trailing slash from SiteTree links if no action present

[xini]

This change helps normalising and canonicalisation of URLs. This picks up #2631 again for SS5.

SiteTree::RelativeLink() is currently the only place in SS where a trailing slash is added to a URL when there is no $action parameter given.

Controller::join_links() already adds slashes between sections of the path, the explicit slash in RelativeLink() is not needed.

Classes that don't use Controller::join_links() at all, e.g. https://github.com/silverstripe/silverstripe-framework/blob/4/src/ORM/PaginatedList.php#L446-L453 use the current request, which never has a trailing slash.

Parent issue

• Remove trailing slash from SiteTree links if no action present #2780

[GuySartorelli]

@xini We've added some acceptance criteria to #2780
Can you please modify this PR to meet those acceptance criteria?

[GuySartorelli]

Rebased, added necessary JS changes, and squashed commits.
Note that this PR will need to be modified once silverstripe/webpack-config#57 has been merged and released in NPM.

[GuySartorelli]

webpack-config changes are now included in this PR

[GuySartorelli]

Failing JS tests are going to be dealt with in silverstripe/silverstripe-admin#1421