Fix broken UI on long filenames (#2215)

If you select a presentation with a very long filename in in Greenlight,
the filename text in the upload dialog wraps at the end and leaves the
label box, leading to a slightly broken looking user interface.

This patch adjusts the style to hide the end of a long filename, ending
with a horizontal ellipsis instead.

app/assets/stylesheets/rooms.scss

@@ -125,4 +125,11 @@
 
 .create-room-button {
   width: 49%;
-}
+}
+
+#presentation-upload-label {
+  overflow: hidden;
+  white-space: nowrap;
+  text-overflow: ellipsis;
+  padding-right: 75px;
+}