List and revoke portal members under Admin → Users

GET /cloud/members asks the portal who it admits to this instance and
marks those with a local user. DELETE /cloud/members/{portal_sub} takes a
membership back. Admin → Users lists the members who have not arrived
yet, with Revoke, which also surfaces a membership left standing when a
local user was deleted while the portal could not be told.

docs/interface/portal.md

@@ -52,6 +52,10 @@ Their account appears under **Admin → Users** with a *Portal* badge. They
 sign in through the portal rather than with a local password. Invitations are
 time-limited; generate a fresh one if it expires.
 
+Somebody admitted by a link gets their local account the first time they open
+the instance. Until then they are listed under **Admin → Users → Pending portal
+members**, where you can revoke the invitation.
+
 ## Manage or end remote access
 
 **Settings → Remote access** shows the connection status and the latest