Fix: SingleLineなMFMが折り返されてしまう (#5109)

src/client/app/common/views/components/misskey-flavored-markdown.vue

 <template>
-<mfm-core v-bind="$attrs" class="havbbuyv" v-once/>
+<mfm-core v-bind="$attrs" class="havbbuyv" :class="{ plain: $attrs['plain-text'] }" v-once/>
 </template>
 
 <script lang="ts">
@@ -17,6 +17,9 @@ export default Vue.extend({
 .havbbuyv
 	white-space pre-wrap
 
+	&.plain
+		white-space pre
+
 	>>> .title
 		display block
 		margin-bottom 4px