Implemented phase 5 for the POC prototype in the Website service.

Services/Website/Sources/Library/Public/Controllers/SessionController.swift

@@ -194,8 +194,13 @@ private extension SessionController {
             )
         }
 
-        let token = await sessions.start(for: try await accounts.account(email: email))
-        var response = Response.redirect(to: returnPath, type: .normal)
+        let account = try await accounts.signIn(email: email)
+        let token = await sessions.start(for: account.user)
+        // A new account picks its display name before going on.
+        var response = Response.redirect(
+            to: account.isNew ? .Route.welcome(returningTo: returnPath) : returnPath,
+            type: .normal
+        )
 
         response.setCookie(SessionStore.cookie(token, isSecure: isSecure))