Support composing multiline (`Shift + Enter`) messages in threaded conversations. Currently, multiline messages work, but can be achieved only via copy/paste. ## Relevant code - [`public/js/vue/thread-message-feed/components/chat-input.vue`](https://gitlab.com/gitlab-org/gitter/webapp/-/blob/85e6ffebcf8e7e0919ec9732fbb0af0d86182c21/public/js/vue/thread-message-feed/components/chat-input.vue#L109-126)