Implement and expose Weight on Boards::EpicListType in GraphQL

Similarly to the EE::BoardListType, we need to expose weight on Epic board lists.

Issue board lists implement total_weight, exposed via a GraphQL field. This is a simple sum of the weights of the issues in the list.

"Weight" in this context means a sum of the unique issues under the epics in a given column, with no double-counting. See #321392 (comment 655304690).

Query that could be possibly used: select sum(issues.weight) from epic_issues join issues on epic_issues.issue_id=issues.id where epic_id in (select distinct id from epics where <epic board conditions>);