Fix #4178: add ability to mass undo tag edits.

Adds checkboxes to the /post_versions index allowing you to select and
undo multiple versions at once.

app/javascript/src/styles/specific/post_versions.scss

@@ -0,0 +1,9 @@
+body.c-post-versions.a-index {
+  #subnav-undo-selected-link {
+    display: none;
+  }
+
+  .post-version-select-column {
+    min-width: 2em;
+  }
+}