eAuth: "Open DM" opens the %groups path on the host's domain.

If I visit a project hosted by ~sartyr and eAuth as ~sarlev-sarsen, the "send message ->" button appears. If I click it, the url that opens is sartyr.rooftopdao.io/apps/groups/some/dm/path, rather than the expected sarlev-sarsen.rooftopdao.io/apps/groups/some/dm/path.

I don't know how easy of a problem this is to solve. AFAIK, web+urbitgraph:// was deprecated a while back so we likely can't use that for opening at the user-defined domain, but perhaps Eyre has some knowledge of the authenticated ship's expected domain that we can pull from the host ship?

Giving this the enhancement tag as it is not a critical feature for the beta release; we can just remove these buttons on the eauth version if need be.