Add setting of a project feature gate

When the first RC of 11.8 is deployed to .com, it comes with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24184. Which allows for projects as feature gates.

Chatops should support this to make it easier to set these feature gates.

Now the current way of setting a feature gate is:

/chatops run feature set my_new_feature true

But in this case we should extent it so we can pass a projects full path and that's send to GitLabs API as JSON param in the HTTP body. I propose something like: /chatops run feature set --project=gitlab-org/gitaly true.

/cc @jacobvosmaer-gitlab