Allow artists to add links to a trackgroup on other platforms

[simonv3]

I've noticed that when an artist shares a release of a new album, they'll share a linktree equivelant with links to all the platforms their album is being released on.

It would be nice to save them a step and let them add links for a trackgroup on the mirlo page, somewhere like :artistId/release/:trackGroupId/share and display something like