Add X-GitLab-Event header for web hooks

Created by: Bugagazavr

What does this MR do? This MR adds to all webhooks. Are there points in the code the reviewer needs to double check?

• https://github.com/Bugagazavr/gitlabhq/blob/1957bbb661f423d0454d3a05ac913ea7af99dabe/app/models/hooks/web_hook.rb#L32

Why was this MR needed? This allow filter incoming hooks without parsing payload, similar feature on github https://developer.github.com/v3/activity/events/types/

What are the relevant issue numbers / Feature requests? nil Screenshots (if relevant) nil