Move an epic to another group via the UI

Problem to solve

Sometimes the scope of an epic changes and is either narrower (only affecting a subgroup) or wider (affecting sibling groups / the parent group) afterwards. The current workflow for those cases is to create a new epic and copy all issues individually, which leads to a poor user experience.

Proposal

We should support moving epics to other groups like we do for issues in the UI. We should allow for moving epics via the already existing Move Issue button in the sidebar. We can rename it to Move Issue/Epic or a generic Move. Epics should be able to be moved to another group or sub-group in this manner. We should also support moving an Epic to a group higher up in the hierarchy.