tasks: add check for missing french locales to the linter

This lint will fail if any locale defined in the code doesn't have
a french localized string.

lib/tasks/lint.rake

@@ -2,6 +2,7 @@ task :lint do
   sh "bundle exec rubocop --parallel"
   sh "bundle exec haml-lint app/views/"
   sh "bundle exec scss-lint app/assets/stylesheets/"
+  sh "bundle exec i18n-tasks missing --locales fr"
   sh "bundle exec brakeman --no-pager"
   sh "yarn lint:js"
 end