Epic Boards - Add Boards navigation item under Epics

What does this MR do?

Adds Board navigation item in left sidebar under Epics.

URL should be of shape groups/gitlab-org/-/epic_boards.

Landing page should be blank with a filter bar. Can only be tested anonymously as currently a logged in user gets redirected to their last issue board. This will be fixed in a separate MR.

Behind :epic_boards feature flag.

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 #231391 (closed)