Show a warning if you are typing a message without a subject

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

src/components/Composer.vue

@@ -83,6 +83,9 @@
 				@keyup="onInputChanged"
 			/>
 		</div>
+		<div v-if="noSubject" class="warning">
+			{{ t('mail', 'This mail does not have a subject yet.') }}
+		</div>
 		<div v-if="noReply" class="warning noreply-box">
 			{{ t('mail', 'Note that the mail came from a noreply address so	your reply will probably not be read.') }}
 		</div>
@@ -223,6 +226,9 @@ export default {
 		isReply() {
 			return !_.isUndefined(this.replyTo)
 		},
+		noSubject() {
+			return this.subjectVal === '' && this.bodyVal !== ''
+		},
 	},
 	beforeMount() {
 		if (this.fromAccount) {