Create UI for accepting an invoice

[michielbdejong]

As we preliminarily decided in cs3org/OCM-API#128 (comment) the string that a user should be able to paste into their EFSS should take the form ocm-invite://<token>@<fqdn>. We should make sure that NC has a UI for this and that our Cypress scripts in dev-stock are able to use it in an ocmstub->nextcloud invite flow (so where NC plays the role of Invite Recipient OCM Server as described in the I-D spec.