[EE] Show epic events on group activity page

What does this MR do?

• epic events are now inclueded in the 'All' tab and a new 'Epic events' tab added to this page
• epic events are not showed on other activity pages (e.g. user) yet
• this MR also removes old "non-join_lateral" query for getting events because this was used only for postgres <9.3 and we now require at least 9.6

Related to https://gitlab.com/gitlab-org/gitlab-ee/issues/13990

Related to #7385 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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