Allow to disable the category/feature inference

This allows to disable the category/feature inference, so that MRs can take advantage of thw two following inferences:

• group -> stage: when a group label is present, automatically sets the corresponding stage label.
• stage -> group (when the stage has only one group): when a stage label is present, and the stage has a single group, automatically sets this single group label.

Expected impact

Per group/project

› grep "# https://gitlab.com/gitlab-org/" dry-run.txt | cut -d '/' -f 5 | sort | uniq -c | sort -r

 102 charts
  67 gitlab-ce
  50 gitlab-ee
  31 omnibus-gitlab
  26 customers-gitlab-com
  14 distribution
  13 create-stage
   6 gitlab-ci-yml
   4 gitter
   4 gitaly
   3 ux-research
   2 security-products
   2 quality
   2 gitlab-services
   2 gitlab-runner
   1 serverless
   1 license-gitlab-com
   1 gitlabktl
   1 gitlab-workhorse
   1 gitlab-development-kit
   1 git
   1 frontend
   1 build

Per label

› grep "/label ~" dry-run.txt | cut -d ' ' -f 2-3 | sort | uniq -c | sort -r                      

 168 ~"devops::enablement"
  51 ~"devops::growth"
  40 ~"devops::create"
  26 ~"devops::configure"
  16 ~"devops::monitor"
  12 ~"group::package"
   7 ~"devops::manage"
   3 ~"devops::verify"
   3 ~"devops::package"
   2 ~"devops::secure"
   1 ~"group::project management"
   1 ~"group::composition analysis"
   1 ~"group::compliance"
   1 ~"group::autodevops and
   1 ~"group::access"
   1 ~"devops::secure" ~"group::composition
   1 ~"devops::plan"
   1 ~"Quality"

Relates to gitlab-org/gitlab-insights#141.