Add todo toggle event

CHANGELOG.md

app/assets/javascripts/header.js

app/assets/javascripts/lib/utils/text_utility.js

app/assets/javascripts/right_sidebar.js

app/assets/javascripts/sidebar.js.es6

app/assets/javascripts/todos.js.es6

app/views/layouts/nav/_dashboard.html.haml

spec/features/todos/todos_spec.rb

spec/javascripts/dashboard_spec.js.es6

spec/javascripts/fixtures/dashboard.html.haml

spec/javascripts/fixtures/header.html.haml

spec/javascripts/fixtures/right_sidebar.html.haml

spec/javascripts/fixtures/todos.json

spec/javascripts/header_spec.js

spec/javascripts/right_sidebar_spec.js