Add overflow scroll to proof tree view.

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

assets/scss/bergamot.scss

@@ -51,6 +51,10 @@
         }
     }
 
+    .bergamot-proof-tree {
+        overflow: auto;
+    }
+
     .bergamot-selector {
         button {
             @include var(background-color, background-color);