i18n: make "plain text" translatable

src/components/post_status_form/post_status_form.vue

@@ -35,7 +35,7 @@
       <div class="visibility-tray">
         <span class="text-format">
           <select v-model="newStatus.contentType" class="form-control">
-            <option value="text/plain">Plain text</option>
+            <option value="text/plain">{{$t('post_status.content_type.plain_text')}}</option>
             <option value="text/html">HTML</option>
             <option value="text/markdown">Markdown</option>
           </select>

src/i18n/messages.js

@@ -392,6 +392,9 @@ const en = {
       unlisted: 'Unlisted - Do not post to public timelines',
       private: 'Followers-only - Post to followers only',
       direct: 'Direct - Post to mentioned users only'
+    },
+    content_type: {
+      plain_text: 'Plain text'
     }
   },
   finder: {