Add columns selector JS component and change Rails view to use it

app/assets/stylesheets/admin.scss

@@ -1258,6 +1258,44 @@ table {
   }
 }
 
+.columns-selector {
+
+  span[class^="icon-"] {
+    vertical-align: middle;
+  }
+}
+
+.columns-selector-wrapper {
+  background: $light-gray;
+  padding: 2rem;
+  border-radius: 10px;
+  margin-bottom: 1rem;
+
+  &.hidden {
+    display: none;
+  }
+
+  .column-selector-item {
+    display: inline-block;
+
+    &.hidden {
+      display: none;
+    }
+
+    label {
+      font-size: 17px;
+      line-height: 1.5rem;
+    }
+  }
+}
+
+.column-selecteable {
+
+  .hidden {
+    display: none;
+  }
+}
+
 // 11. Newsletters
 // -----------------