Externalize strings

Also some cleanup. Work on issue #41.

src/components/messages/MessageIncomingFile.vue

@@ -3,7 +3,7 @@
     <div class="bubble">
       <div class="original-message" v-if="inReplyToText">
         <div class="original-message-sender">
-          {{ inReplyToSender || "Someone" }} said:
+          {{ $t('message.user_said', {user: inReplyToSender || "Someone"}) }}
         </div>
         <div
           class="original-message-text"
@@ -11,14 +11,14 @@
         />
       </div>
       <div class="message">
-        <span>File: </span>
+        <span>{{ $t('file_prefix') }}</span>
         <span
           style="cursor: pointer"
           @click.stop="$emit('download')"
           v-html="linkify($sanitize(messageText))"
         />
         <span class="edit-marker" v-if="event.replacingEventId()"
-          >(edited)</span
+          >{{ $t('edited') }}</span
         >
       </div>
     </div>