FE backport of group boards to reduce CE conflicts

app/assets/javascripts/api.js

app/assets/javascripts/boards/boards_bundle.js

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

app/assets/javascripts/boards/components/board_new_issue.js

app/assets/javascripts/boards/components/issue_card_inner.js

app/assets/javascripts/boards/components/modal/footer.js

...assets/javascripts/boards/components/new_list_dropdown.js

...ets/javascripts/boards/components/sidebar/remove_issue.js

app/assets/javascripts/boards/models/issue.js

app/assets/javascripts/boards/models/label.js

app/assets/javascripts/boards/models/list.js

spec/javascripts/api_spec.js