Add view to group allowing visibility into projects shared with the group

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/14626

Currently, if a project chooses to share with a group there is no visibility in to this link from the group itself. It would be nice to have a page in the group that will show any project shares.

The use-case from the customer is that they have groups of users from their customer companies. They then share projects with that group based on the company's license. For auditing purposes they want to be able to look in one place to verify that the company's users have access to the projects they are paying for.

A quick glance at the model shows that we have a shared_projects relationship that should make this very doable. We just need to list these on a new page.

cc/ @rymai @rdavila What do you think? Does this make sense?