Notification widget for work items

What does this MR do and why?

Notification widget will add the ability to subscribe to notifications of a work item record. It will be visible to following items:

• Tasks
• Objectives
• Key results

Screenshots or screen recordings

Without modal	With modal
Screen_Recording_2023-03-30_at_5.54.19_PM	Screen_Recording_2023-03-30_at_5.55.16_PM

How to set up and validate locally

Prerequisites: Enable the following feature flags to create objects supporting this feature using gdk rails console:

1. Feature.enable(:work_items_mvc)
2. Feature.enable(:work_items_mvc_2)
3. Feature.enable(:okrs_mvc, Project.find_by_full_path('gitlab-org/gitlab-test'))

Steps:

1. Go to any Project > Issues > List
2. Create an new Objective
3. Go to Objective detail page
4. Click on the right top kebab menu to see the Notification toggle
5. Use the toggle to turn on or turn off the notifications

On similar lines, a key result can be added to the objective in the Child objectives and key results section. And a task comes under an issue Tasks section.

In both, there is an option to add task/key result.

Steps to check with modal:

1. Go to any Project > Issues > List
2. Create an Objective or Issue
3. Go to Objective/Issue detail page > Child objectives and key results/Tasks section
4. Add a Key result/Task
5. Click on the newly added item
6. Click on the right top kebab menu to see the Notification toggle
7. Use the toggle to turn on or turn off the notifications

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #389444 (closed)