When running in open mode invites can be freely generated by accessing
/create-invite. This displays an HTML page which creates and displays an
invite to the user.
This commit adds an additional way of creating invites in open mode. A
POST request can be sent to the same /create-invite endpoint with the
Accept header set to application/json. This returns a JSON response
which contains an invite url.
The purpose of this change is to make automatic invite generation easier
in SSB clients.
ab664aafc3