List epics on epic boards ordered by relative position

Epics on epic boards should be ordered by its relative position (similar to issue boards).

• this relative position should be scoped to boards (e.g. an epic may have different relative position on different boards)
• this board relative position is unrelated to the epic's existing relative_position attribute which is used in epic tree ordering (IOW we should not reuse this attribute)

To implement this feature, the steps might be:

• add a new table for storing epic relative positions per board
• the model can re-use RelativePositioning concern
• when listing epics in a board list (#233436 (closed)), perhaps we can use left join to join epics with its relative position (there may be null/missing positions for epics) and order epics by this relative position