Add `Invite user` button and modal to organization users page

Proposal

We need a Invite user button added to Organization -> Manage -> Users. This button should open a modal that allows inviting a user. See &13471 for designs.

Implementation guide

1. Add New user button and modal to app/assets/javascripts/organizations/users/components/app.vue#L92
2. Use API built in #455122. If not yet done create follow-up issue to implement API call.