Add todo quick action to work items

What does this MR do and why?

Related to #412277 (closed)

1. Adds quick actions for toggling todos in a work item:

• /todo -> Adds a todo for the current user, available only if no pending todos are present
• /done -> Marks todo as done, available only if a pending todo is present

2. Fixes todos creation for work items to use the correct class in the target_type column.

Screenshots or screen recordings

Screen_Recording_2023-06-22_at_15.52.02

How to set up and validate locally

1. In rails console enable flags and create a task

# Enable FFs
Feature.enable(:work_items_mvc)

# Create task
user, project = User.first, Project.first
task_type_id = WorkItems::Type.find_by(name: 'Task').id
task = WorkItem.create!(title: 'task 1', author: user, project: project, work_item_type_id: task_type_id, issue_type: :task)
task.id # take note of its ID

2. Visit the task at https://gdk.test:3000/gitlab-org/gitlab-test/-/work_items/<task_id>
3. Verify that /done action is not available as there are no pending todos
4. Verify that /todo quick action is available and adds a todo (reloading the page should turn the todo icon blue)
5. Once the work item has a pending todo the /done action should be available and mark the todo as done.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.