Update master with 2.4.0

CHANGELOG.md

CONTRIBUTORS.md

build/dev-server.js

build/webpack.base.conf.js

config/index.js

package.json

src/App.js

src/App.scss

src/App.vue

src/boot/after_store.js

src/boot/routes.js

src/components/account_actions/account_actions.vue

src/components/attachment/attachment.js

src/components/attachment/attachment.vue

src/components/chat_list/chat_list.vue

src/components/chat_message/chat_message.vue

src/components/domain_mute_card/domain_mute_card.vue

src/components/emoji_input/emoji_input.js

src/components/emoji_input/emoji_input.vue

src/components/export_import/export_import.vue