user upgrades: update upgrade landing page.

* Add a frequently asked questions section.
* Add nicer looking upgrade buttons.
* Format the page nicer.
* Prevent users from attempting invalid upgrades on users that are
  already Platinum or above.

app/javascript/src/styles/base/020_base.scss

@@ -118,6 +118,17 @@ table tfoot {
   margin-top: 2em;
 }
 
+details {
+  border-bottom: 1px solid var(--details-border);
+
+  summary {
+    cursor: pointer;
+    user-select: none;
+    outline: none;
+    line-height: 2em;
+  }
+}
+
 .fineprint {
   color: var(--muted-text-color);
   font-style: italic;