Epic templates pre-populated with issues

This is different from a description template, where when you create the object (issue/mr), it has pre-populated text.

In this case, when you create an epic, issues are also pre-created with titles, and already attached to the epic.

Design-wise, we'd need to figure out:

• Where do you manage these templates? Probably source-controlled similar to group level templates: https://docs.gitlab.com/ee/user/group/custom_project_templates.html
• Where do you specify the template when creating an epic? Currently to create an epic, there isn't a dedicated web form. So we would need to design a way to specify that template.