Add server creation UI + user-server access assignment (step 3)

New admin-only page at /servers-settings (templates/servers.html) lists
servers and creates new ones via POST /servers (ID/name/interface/
addresses/port, key pair generated server-side). Nav gets a "Servers"
link.

templates/users_settings.html gains a multi-select "Server Access"
field wired to the server_ids support added to create-user/update-user
in the previous commit, so admins can now actually assign non-admin
users to specific servers through the UI.

templates/base.html

@@ -139,6 +139,14 @@
 
 
                         <li class="nav-header">SETTINGS</li>
+                        <li class="nav-item">
+                            <a href="{{.basePath}}/servers-settings" class="nav-link {{if eq .baseData.Active "servers" }}active{{end}}">
+                                <i class="nav-icon fas fa-server"></i>
+                                <p>
+                                    Servers
+                                </p>
+                            </a>
+                        </li>
                         <li class="nav-item">
                             <a href="{{.basePath}}/global-settings" class="nav-link {{if eq .baseData.Active "global-settings" }}active{{end}}">
                                 <i class="nav-icon fas fa-cog"></i>