Make media modal be aware of multi-touch actions

Originally the media viewer would think every touch is a swipe (one-finger
touch event), so we would encounter the case where a two-finger scale event
would incorrectly change the current media. This is now fixed.

src/components/media_modal/media_modal.vue

@@ -13,6 +13,7 @@
       :title="currentMedia.description"
       @touchstart.stop="mediaTouchStart"
       @touchmove.stop="mediaTouchMove"
+      @touchend.stop="mediaTouchEnd"
       @click="hide"
       @load="onImageLoaded"
     >