Limit co-organiser invitations to max 8 co-organisers

[mariobehling]

Currently an unlimited number of co-organisers can be invited. We need to limit this to 8 co-organisers.

[hongquan]

Applied rate-limiting for the API.
Ref:
https://blog.nginx.org/blog/rate-limiting-nginx
http://nginx.org/en/docs/http/ngx_http_limit_req_module.html#limit_req

[rahulbansal3005]

Hi @mariobehling I would like to work on this task.