Remove labels when moving epic between epic board lists

Summary

When moving an epic from a label list to open, the label is not removed, so upon refresh, all the moved epics go back to the list you just moved them out of. To send an epic back to the open list users have to manually remove the label.

Steps to reproduce

1. Go to the epic board
2. Move an epic from "open" list to a labeled list
3. Move the epic to "closed" list
4. Move the epic back to the "open" list

What is the expected behavior

The epic is in "open" list

What is the current behavior

The epic is in the labeled list, the label is kept on the epic.

Possible fixes

Fix in Boards::Epics::MoveService