Remove usages of and references to <input type="datetime">

It has been removed from the HTML specification: https://github.com/whatwg/html/issues/336
Only Presto Opera ever implemented a special UI for it.

[skip sauce]

docs/content/reboot.md

@@ -225,7 +225,7 @@ These changes, and more, are demonstrated below.
 
     <p>
       <label for="time">Example temporal</label>
-      <input type="datetime" id="time">
+      <input type="datetime-local" id="time">
     </p>
 
     <p>