Epic Boards - Fix mentions of issues to epics

What does this MR do?

Fixes text on board list to reflect the type of issuable. (Showing all issues / Showing all epics). Refactor IssuCount component to be more agnostic of its issuable type.

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)
Update board scope	!55815 (merged)
Drag & Drop epic	!56170 (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 _____.
• 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