Project services and JIRA documentation clean up

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/176

This was first aiming to fix the JIRA references to EE, but as I was going through it, I decided to revamp more.

@Haydn once merged here and then Valery merges CE into EE it should be fixed in doc.gitlab.com as well.

@dblessing the docs state that we support JIRA 6.x. I see that 7.x is out, should the docs be amended? Also we seem to have a place to document services (http://doc.gitlab.com/ce/project_services/project_services.html), but the JIRA doc is under integration :/ I'd like it to have it moved under `project_services/`, but I guess most customers know the current URL better.

See merge request !2345