Use gitlab-dangerfiles

I looked into the https://gitlab.com/gitlab-org/gitlab-dangerfiles, and it looks like we've started to migrate the gitlab project to using this.

I assume there are more efforts to migrate plugins out of gitlab and into gitlab-dangerfiles, and when this happens it'll be super useful for us here and we should start using it.

The documentation doesn't specify it, but when we add it, the Gemfile should probably look like this:

gem 'gitlab-dangerfiles', require: false

And the Dangerfile should look like this:

require 'gitlab-dangerfiles'

gitlab_dangerfiles = Gitlab::Dangerfiles::Engine.new(self)
# ...

This will keep from requiring code that's not needed needed when not used.