Move BoardService logic to boardsStore

This is part of refactoring issue boards to Vuex (&1278 (closed)).

Move any logic that happen in BoardService or BoardServiceEE (ee/app/assets/javascripts/boards/services/board_service.js) to the boardsStore.

Example

diff --git a/ee/app/assets/javascripts/boards/services/board_service.js b/ee/app/assets/javascripts/boards/services/board_service.js
index c7f64ed3f67..46f9c1990ec 100644
--- a/ee/app/assets/javascripts/boards/services/board_service.js
+++ b/ee/app/assets/javascripts/boards/services/board_service.js
@@ -7,7 +7,7 @@ export default class BoardServiceEE extends BoardService {
   }

   recentBoards() {
-    return axios.get(this.recentBoardsEndpoint);
+    return boardsStore.fetchRecentBoards();
   }

   createBoard(board) {