Limit number of hooks triggered by a single push

@stanhu wrote in https://gitlab.com/gitlab-org/gitlab-ce/issues/65804#note_203405978:

Do we really want to be firing one hook per branch update? It seems to me that once we exceed some threshold (e.g. 10 branch or tag pushes), we should just aggregate the push data into a single field and send one event per hook/service. The activity feed also shouldn't be littered with 10,000 push events; maybe 1 post should be adequate there too.

Expected outcome

Don't fire hooks/services if there are more than 3 branches/tags in a single push.

Documentation

• Update the documentation about webhooks and push events to add information about the limit: https://docs.gitlab.com/ee/user/project/integrations/webhooks.html#push-events
• Add information about services and push event limit: https://docs.gitlab.com/ee/user/project/integrations/project_services.html