Add metrics dashboard json schema validator

Gemfile

Gemfile.lock

lib/gitlab/metrics/dashboard/validator.rb

lib/gitlab/metrics/dashboard/validator/client.rb

lib/gitlab/metrics/dashboard/validator/custom_formats.rb

lib/gitlab/metrics/dashboard/validator/errors.rb

...tlab/metrics/dashboard/validator/post_schema_validator.rb

locale/gitlab.pot

...s/lib/gitlab/metrics/dashboard/duplicate_id_dashboard.yml

...xtures/lib/gitlab/metrics/dashboard/invalid_dashboard.yml

spec/lib/gitlab/metrics/dashboard/validator/client_spec.rb

...gitlab/metrics/dashboard/validator/custom_formats_spec.rb

spec/lib/gitlab/metrics/dashboard/validator/errors_spec.rb

...metrics/dashboard/validator/post_schema_validator_spec.rb

spec/lib/gitlab/metrics/dashboard/validator_spec.rb