webadmin: combine "List Users" & "Add User"

Let add users via the list of users in the similar manner it’s
done for networks. It’s evidently an extra step to add a user,
but on the other hand it gives a nice overview of the users
when adding new ones, and leads to less clutter in the sidebar.

modules/data/webadmin/tmpl/listusers.tmpl

@@ -9,7 +9,7 @@
 				<table>
 					<thead>
 					<tr>
-						<td>Action</td>
+						<td>[<a href="<? VAR URIPrefix TOP ?><? VAR ModPath TOP ?>adduser">Add</a>]</td>
 						<td>Username</td>
                         <td>Networks</td>
 						<td>Clients</td>