Add button to show tags on smaller screens (#529)

* Implement tag modal

* Improve header controls responsiveness

* Improve modal styles

* Cleanup

bookmarks/styles/base.scss

@@ -50,14 +50,20 @@ section.content-area {
     border-bottom: solid 1px $border-color;
     display: flex;
     flex-wrap: wrap;
+    column-gap: $unit-6;
     padding-bottom: $unit-2;
     margin-bottom: $unit-4;
 
     h2 {
+      flex: 0 0 auto;
       line-height: 1.8rem;
-      margin-right: auto;
       margin-bottom: 0;
     }
+
+    .header-controls {
+      flex: 1 1 0;
+      display: flex;
+    }
   }
 }