Add "epic" to Advanced Global Search

Problem to solve

Currently users can not find epics using advanced global search.

Intended users

Personas are described at https://about.gitlab.com/handbook/marketing/product-marketing/roles-personas/

User experience goal

Users should be able to find Epics using Global Search.

Proposal

Add Epics to Global Search. Example of Google index Epics

!-- How are we going to solve the problem? Try to include the user journey! https://about.gitlab.com/handbook/journeys/#user-journey -->

Further details

Permissions and Security

Epics should only be available to those with permissions.

Documentation

Availability & Testing

What does success look like, and how can we measure that?

What is the type of buyer?

Is this a cross-stage feature?

Links / references