(nordigen) check server status before linking accs (#742)

Related to:
https://github.com/actualbudget/actual/issues/724#issuecomment-1455160250

Depends on https://github.com/actualbudget/docs/pull/126 to be merged
first.

Two changes here:
1. show "link account" only if actual-server is used (user is "online");
2. allow linking accounts only if Nordigen is configured (using new API
to get the status for it);

Also ported the `CreateAccount` modal to a functional component.