Migrate Add To Do button to widget

...ripts/design_management/components/design_todo_button.vue

...ts/sidebar/components/todo_toggle/sidebar_todo_widget.vue

app/assets/javascripts/sidebar/mount_sidebar.js

app/assets/javascripts/sidebar/utils.js

...red/components/sidebar/todo_toggle/todo_button.stories.js

...vue_shared/components/sidebar/todo_toggle/todo_button.vue

...cripts/vue_shared/components/sidebar/todo_toggle/utils.js

doc/development/fe_guide/storybook.md

...d/design_management/components/design_todo_button_spec.js

...idebar/components/todo_toggle/sidebar_todo_widget_spec.js

...rontend/vue_shared/components/sidebar/todo_button_spec.js