Epic board sidebar - Edit subscribed state [RUN AS-IF-FOSS]

What does this MR do?

Adds ability to turn notifications on or off from epic board sidebar.

Requires :epic_boards feature flag.

Progress of Epic boards feature development

Previous iterations	MR link
Index page	!53100 (merged)
Empty lists	!54169 (merged)
Epics in lists	!54528 (merged)
Multiple epic boards switcher	!54399 (merged)
Create epic board	!54642 (merged)
Display epic reference in card	!55028 (merged)
Add label list	!55011 (merged)
Collapse list and polish	!55456 (merged)
MVC1 bug fixes	!55709 (merged)
Drag & Drop epic	!56170 (merged)
Fix mentions of issues to epics	!56292 (merged)
Update board scope	!55815 (merged)
Delete epic board	!57186 (merged)
Edit epic labels	!57973 (merged)
Edit epic title	!58727 (merged)
Edit epic confidentiality	!58968 (merged)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this feature is behind a feature flag.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #231401 (closed)