Add subscribe and unsubscribe quick actions to work items

What does this MR do and why?

Add /subscribe and /unsubscribe quick actions for work items.

Related to: #420796 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Subscribe-Unsubscribe-quick-action

How to set up and validate locally

1. In rails console, enable the feature flag:

Feature.enable(:krs_mvc)

2. Navigate to your test project.
3. On the left sidebar, select Plan > Issues
4. In the upper-right corner, next to New issue, select the down arrow and then select New objective.
5. Select New objective again.
6. Select Create objective.
7. Log in as another user.
8. In the comment box, type /subscribe:
9. This user should now be subscribed to this work item. You can confirm this by refreshing the page and confirming that notifications are on by clicking on the menu next to 'Close Objective' in the top-right. The notifications toggle should be On.
10. Typing /unsubscribe should result in the opposite.

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.