Epic delete deletes all epics

Summary

When deleting an epic, it deletes all child epics as well.

Steps to reproduce

1. Create epic called one
2. Create epic called two and make one a child of two
3. Create epic called three and make two a child of three
4. Go to epic two, edit the description and click delete.

What is the current bug behavior?

5. Notice that Epic two and one are now gone. If you got to the curl you get a 404.

What is the expected correct behavior?

Not completely sure, probably a combination of:

1. Refusing because there are child epics
2. Change the modal on delete to say that it will also remove child epics
3. Only delete the currently active epic and break the chain.

Output of checks

This happens on Gitlab.com.