Boards - Fetch list issues via GraphQL

As part of Boards refactoring, issues should be fetched using GraphQL endpoint (see #229138 (closed) for backend), through a VueX action.

Some refactoring of the issue model will be required. The use of models being an uncommon pattern in GitLab, we should consider a refactoring getting rid of the model by replacing it with mixins or util functions.

Query is a combination of: #230903 (closed) and #229143 (closed) (fetch issues under board by specifying a single list).

This VueX action should be called behind feature flag :graphql_board_lists