css Invalid property value

Apologies, I seem to have introduced a bug here: https://github.com/hypothesis/client/pull/2461

It should be:

```
border-width: 0 0 1px 0;
```

rather than:

```
border: 0 0 1px 0;
```

src/styles/mixins/molecules.scss

@@ -34,7 +34,7 @@
     // Give a little more space so that the border appears centered
     // between cards
     padding-bottom: 1.5em;
-    border: 0 0 1px 0;
+    border-width: 0 0 1px 0;
     border-bottom-style: solid;
     border-bottom-color: var.$grey-2;
     box-shadow: none;