Gitaly Server info for admin panel

Implements the client side for gitlab-org/gitaly#819. Which is a server
info command. This checks the server version and git binairy version on
the server.

A small UI was added for administrators, so they can check the status of
the Gitaly server. This is done for each storage the monolith knows.

Because of this commit, gitlab-org/gitlab-ce!15580 is now closed. That
MR removed the Git version too, but didn't replace it with anything.