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

@@ -103,7 +103,7 @@
             :disabled="posting"
             class="form-post-body"
             @keydown.meta.enter="postStatus(newStatus)"
-            @keyup.ctrl.enter="postStatus(newStatus)"
+            @keydown.ctrl.enter="postStatus(newStatus)"
             @input="resize"
             @compositionupdate="resize"
             @paste="paste"