Epic Boards - Collapse list and polish [RUN AS-IF-FOSS]

What does this MR do?

• Display epics count and epic icon in header
• Save collapsed state of list on localStorage - This is temporary until the mutation exists on the API
• Hide new button in header
• Hide weight in header
• Hide Delete Board option in board selector dropdown

We are hiding actions that will be available in future iterations of epic boards. More details in the epic: &2864

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)

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• 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 #321770 (closed)