Add list subscriptions to namespaces call to enable frontend to show subscription state correctly.
Resolves https://github.com/go-vikunja/frontend/issues/75
Reviewed-on: vikunja/api#1254
Reviewed-by: konrad <k@knt.li>
Co-authored-by: Luca Bernstein <luca@lucabernstein.com>
Co-committed-by: Luca Bernstein <luca@lucabernstein.com>