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/dcim/rack.html

@@ -268,7 +268,7 @@
                             </td>
                             <td>
                                 {{ resv.description }}<br />
-                                <small>{{ resv.user }} &middot; {{ resv.created }}</small>
+                                <small>{{ resv.user }} &middot; {{ resv.created|annotated_date }}</small>
                             </td>
                             <td class="text-right noprint">
                                 {% if perms.dcim.change_rackreservation %}