Migrate issue board store logic to Vuex

This is part of refactoring issue boards to Vuex (&1278 (closed)) and needs to happen after #28453 (closed) and gitlab-foss#61622 (closed)

Move any logic that happens in app/assets/javascripts/boards/stores/boards_store.js to the newVuex store (ideally one function at a time).

Example

  addList(listObj, defaultAvatar) {
-   this.state.lists.push(list);
+   this.vuexStore.dispatch('addList', list);

(https://gitlab.com/gitlab-org/gitlab-ce/blob/b15a78dfdd497848a0fd763185e79b0361da1dbb/app/assets/javascripts/boards/stores/boards_store.js#L36)