Closes #5503: ISO 8601 date in UI and alternative format as tooltip

With this commit all dates in the UI are now consistently displayed.

I changed the long date format as suggested by @xkilian and confirmed by my own
research.

* DATETIME_FORMAT
 * Before July 20, 2020 4:52 p.m.
 * Now 20th July, 2020 16:52

"20th July, 2020" would be spoken as "the 20th of July, 2020" but the "the" and
"of" are never written.

The only exception is `object_list.html`. I tried it but there it does not
work so easily because the dates are passed to Jinja as SafeString.

netbox/templates/ipam/aggregate.html

@@ -54,7 +54,7 @@
                 </tr>
                 <tr>
                     <td>Date Added</td>
-                    <td>{{ object.date_added|placeholder }}</td>
+                    <td>{{ object.date_added|annotated_date|placeholder }}</td>
                 </tr>
                 <tr>
                     <td>Description</td>