[maven] improve usablity of maven package repository
Problem to solve
The maven package feature is rather new and feels a bit unpolished and hard to use.
- show URL and login instructions on the "packages" page. Similar to "How to use the Container Registry" for the Registry.
- make use of real maven URLS for packages page when downloading artifacts. Currently it has urls like "https://...gitlab.host/group/project/-/package_files/4/download". A good example would be "http://central.maven.org/maven2/io/crnk/crnk-core/2.6.20180522184741/crnk-core-2.6.20180522184741.jar".
- repositories should have meaningful urls. In that regard they should make use of project/group names rather than project ids like api/v4/projects/2/packages/maven. That "2" is rather confusing when working with multiple repositories.
- make maven endpoint discoverable: when acessing api/v4/projects/2/packages/maven from the browser, it should be possible to navigate that endpoint, see sub directories, etc.
- created dedicated ticket for authentication issues (https://gitlab.com/gitlab-org/gitlab-ee/issues/10749)
- packages table listing all artifacts should only show name and latest version instead of all versions on many pages. Detail page should then show the list of all versions.