Refactor of issue boards to work with Vue2

app/assets/javascripts/boards/boards_bundle.js.es6

app/assets/javascripts/boards/components/board.js.es6

app/assets/javascripts/boards/components/board_card.js.es6

app/assets/javascripts/boards/components/board_list.js.es6

...sets/javascripts/boards/components/board_new_issue.js.es6

...ts/javascripts/boards/components/new_list_dropdown.js.es6

...javascripts/boards/mixins/sortable_default_options.js.es6

app/assets/javascripts/boards/stores/boards_store.js.es6

app/assets/stylesheets/pages/boards.scss

app/views/projects/boards/_show.html.haml

app/views/projects/boards/components/_blank_state.html.haml

app/views/projects/boards/components/_board.html.haml

app/views/projects/boards/components/_board_list.html.haml

app/views/projects/boards/components/_card.html.haml

app/views/projects/boards/index.html.haml

app/views/projects/boards/show.html.haml