Add a usage_limit parameter to the API.
This limits the number of times a setup key
can be used.
usage_limit == 0 indicates the the usage is inlimited.
This PR brings user invites logic to the Management service
via HTTP API.
The POST /users/ API endpoint creates a new user in the Idp
and then in the local storage.
Once the invited user signs ups, the account invitation is redeemed.
There are a few limitations.
This works only with an enabled IdP manager.
Users that already have a registered account can't be invited.
