Add multisite compat

On a multisite, any user can login from any site on the multisite and be able to gain access to whatever site's dashboard per their role on that particular site.

Need to account for someone logging in locally via Site A as an author, editor, etc even though they have admin on Site B.

Start by reviewing how multisite login establishes the current site and how dashboard access is checked against per-site roles. Verify the behavior for a user who is an author or editor on Site A but an administrator on Site B, and confirm that login from either site grants only that site's appropriate dashboard access.

Written by the indexing model from the issue text.