Add a `description` entry to job definition in CI configuration

Description

Because build name is usually a short string that rarely describes a purpose of the CI job adequately, it may be a good idea to add a description keyword to a build definition in .gitlab-ci.yml.

rspec:
  script: rspec
  description: 'Unit tests for GitLab'

rubocop:
   script: rubocop
   description: 'Static code analysis'

Proposal

Add description keyword, and if present show it in the tooltip in pipelines graph and in build details page.

Links / references

https://gitlab.slack.com/archives/questions/p1479474951004415

/cc @ayufan @markpundsack @selfup @cperessini