Move BoardService logic to boardsStore
Moves all logic out of the issue boards service and into the store. This is a preparation for deleting the service and eventually using Vuex actions. Deleting is a separate issue because it requires updating a lot of places that are not touched here (see https://gitlab.com/gitlab-org/gitlab-ce/issues/61621).
Edited by 🤖 GitLab Bot 🤖