[RUN-AS-IF-FOSS] Epic Boards index page

What does this MR do?

• Load epic board data on epic_boards page
• Add and load index javascript
• Prevent redirection to last issue board when logged in

Backend:

• Remove default issue creation from the Boards::ListService and into the objects that call it
• Make board logic more extensible for other board types (there is still work to do here)

Behind feature flag epic_boards.

To test, point your browser to: http://gdk.test:3000/groups/group/-/epic_boards. Only the index action works, but you can also test that issue boards still work:

• http://gdk.test:3000/groups/group/-/boards
• http://gdk.test:3000/groups/group/-/boards/1
• http://gdk.test:3000/group/project_path/-/boards
• http://gdk.test:3000/group/project_path/-/boards/1

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 #294043 (closed)