Update boostrap to latest 4.3.1

Co-authored-by: Graham Christensen <graham@grahamc.com>
... but just fixing up merge conflicts from the introduction of flakes
and the removal of the Jobs table.

src/root/users.tt

@@ -24,8 +24,8 @@
   </tbody>
 </table>
 
-<p><a class="btn" href="[% c.uri_for(c.controller('Root').action_for('register')) %]">
+<a class="btn btn-primary" href="[% c.uri_for(c.controller('Root').action_for('register')) %]">
   <i class="icon-plus"></i> Add a new user
-</a></p>
+</a>
 
 [% END %]