Add styling for the file input area

The file input area has a transparent border. When a file is dragged
above that area, the border turns blue. This helps the user understand
that this is the correct drop-off area.

stylesheets/_global.scss

@@ -136,6 +136,8 @@ button.back {
 .file-input {
   position: relative;
   margin-right: 10px;
+  box-sizing: content-box;
+  border: 1px transparent solid;
   .choose-file {
     cursor: pointer;
   }
@@ -178,6 +180,10 @@ button.back {
   }
 }
 
+.dropoff {
+  border: 1px #2090ea dashed;
+}
+
 img.emoji {
    height: 1em;
    width: 1em;

stylesheets/manifest.css

@@ -117,7 +117,9 @@ button.back {
 
 .file-input {
   position: relative;
-  margin-right: 10px; }
+  margin-right: 10px;
+  box-sizing: content-box;
+  border: 1px transparent solid; }
   .file-input .choose-file {
     cursor: pointer; }
   .file-input .paperclip {
@@ -151,6 +153,9 @@ button.back {
     color: #fff;
     text-align: center; }
 
+.dropoff {
+  border: 1px #2090ea dashed; }
+
 img.emoji {
   height: 1em;
   width: 1em;