Ask shared accounts together, and about files only when Files opens

At sign-in the files, contacts and calendar stores each asked every shared
account a question, one account after another: a request apiece before the
reader had opened any of those views.

Files now only works out at sign-in whether it is available. Which shared
accounts hold files is asked when the Files view or the file picker opens,
which the Files view already did on every visit. Shared address books and
calendars are asked for in one request, and the calendar store loads its
calendars, identities and shared calendars side by side.

web/src/views/files/FilesTree.tsx

@@ -32,6 +32,7 @@ async function refreshShares(force = false): Promise<void> {
     return;
   }
   await useFiles.getState().init();
+  await useFiles.getState().discoverShared();
 }