Trainee maintainers count as reviewers, but should get more reviews than
most reviewers, as they need practice to become a maintainer. This makes
them three times as likely to be picked (compared to another reviewer;
compared to before, it's roughly twice as likely at the current numbers
for each).

Also, switch to `Array#sample` because I think it looks nicer.

This extends Danger so it informs MR authors that single codebase merge
requests need to be approved by a specific group of engineers.

Co-authored-by: Yorick Peterse <yorickpeterse@gmail.com>

Template and process improvements for documentation

See merge request gitlab-org/gitlab-ce!24315

This introduces additional method for linting,
to reduce the complexity of `lint_commits`.

# Conflicts:
#   .gitlab/merge_request_templates/Documentation.md