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/media_upload/media_upload.js

@@ -45,20 +45,6 @@ const mediaUpload = {
         this.$emit('all-uploaded')
       }
     },
-    fileDrop (e) {
-      if (e.dataTransfer.files.length > 0) {
-        e.preventDefault() // allow dropping text like before
-        this.multiUpload(e.dataTransfer.files)
-      }
-    },
-    fileDrag (e) {
-      let types = e.dataTransfer.types
-      if (types.contains('Files')) {
-        e.dataTransfer.dropEffect = 'copy'
-      } else {
-        e.dataTransfer.dropEffect = 'none'
-      }
-    },
     clearFile () {
       this.uploadReady = false
       this.$nextTick(() => {