Add visual indication for drag&dropping files, make dropzone bigger

When you drag a file over a post-status-form it now highlights the area as a drop zone, drop zone size has been increased from just text-box to whole form

I also removed dangerous duplicate logic, no need for media upload button to also listen to drag&drop events

src/components/post_status_form/post_status_form.vue

@@ -6,12 +6,12 @@
     <form
       autocomplete="off"
       @submit.prevent="postStatus(newStatus)"
-      @dragover.prevent.capture="fileDrag"
+      @dragover.prevent="fileDrag"
     >
       <div
         v-show="showDropIcon"
         class="drop-indicator icon-upload"
-        @dragleave.capture="fileDragStop"
+        @dragleave="fileDragStop"
         @drop.stop="fileDrop"
       />
       <div class="form-group">