Review server permissions at the graph validation level

[HerbCaudill]

In servers.test.ts there are a bunch of tests demonstrating what a server can and cannot do, but it seems like those permissions are enforced by a combination of two things: (1) if a server isn't an admin (and it probably shouldn't ever be) then it can't add any links to the graph that are admin-only; and (2) there are checks in the Team public API that will throw if called by a server. But for example I think a server could add an 'ADD_DEVICE' link to the chain directly (without going through the public API).

As a side note we should probably enforce that a server can't ever be an admin.