Update todo and notifications for work items

What does this MR do and why?

Use the new two-button layout used on issuables for work items.

This work is behind a feature flag (:notifications_todos_buttons) and a followup will be necessary to remove that later.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Check out this branch in your GDK
2. turn on the two relevant feature flags:
   1. echo "Feature.enable(:work_items_mvc_2)\nFeature.enable(:notifications_todos_buttons)" | gdk rails c
3. visit a work item (task or OKR for example).
   • canned Task your GDK may already contain: http://localhost:3000/flightjs/Flight/-/work_items/3 / http://gdk.test:3000/flightjs/Flight/-/work_items/3
4. You will see that the new two-button todo and notification management is in place and that notification management is no longer present in the "More options" menu for the work item. The Todo button has also been fixed to use the btn-icon class.

Related to #431932 and #424397 (closed)