Scaffold UI elements for minimal version

app/assets/javascripts/ci_variable_list/ci_variable_list.js

app/assets/stylesheets/framework/ci_variable_list.scss

app/controllers/groups/variables_controller.rb

app/controllers/projects/variables_controller.rb

app/helpers/ci_variables_helper.rb

app/serializers/group_variable_entity.rb

app/serializers/variable_entity.rb

app/views/ci/variables/_content.html.haml

app/views/ci/variables/_header.html.haml

app/views/ci/variables/_variable_row.html.haml

...elogs/unreleased/13784-validate-variables-for-masking.yml

locale/gitlab.pot

spec/features/group_variables_spec.rb

spec/features/project_variables_spec.rb

spec/fixtures/api/schemas/variable.json

spec/javascripts/ci_variable_list/ci_variable_list_spec.js

spec/support/features/variable_list_shared_examples.rb