move_topic: Improve the style for dropdown toggle button.

Earlier, the `stream_header_colorblock` wasn't properly styled
with the dropdown toggle button from a UI perspective. This was so
because they had a few space between them due to inconsistencies
in their border radius.

This commit adds an border-radius property to the move topic
dropdown toggle button to eliminate the above issue and
improve it's overall look.

static/styles/popovers.css

@@ -652,6 +652,12 @@ ul {
         overflow: visible;
     }
 
+    /* Override the default border-radius to properly align
+       the button corners with `stream_header_colorblock`. */
+    .dropdown-toggle {
+        border-radius: 1px 4px 4px 1px !important;
+    }
+
     .stream_header_colorblock {
         margin-bottom: 20px;
     }