Skip PresharedKey generation (#271)

Helpful for those who already have users deployed.

Enter `-` in the Preshared Key field at user creation time to skip its
creation.

The template conf takes care of the rest.

Fixes issue #235

templates/base.html

@@ -206,7 +206,7 @@
                                 <summary><strong>Public and Preshared Keys</strong>
                                     <i class="fas fa-info-circle" data-toggle="tooltip"
                                        data-original-title="If you don't want to let the server generate and store the
-                                       client's private, you can manually specify its public and preshared key here
+                                       client's private key, you can manually specify its public and preshared key here
                                        . Note: QR code will not be generated">
                                     </i>
                                 </summary>
@@ -220,7 +220,7 @@
                                     <label for="client_preshared_key" class="control-label">
                                         Preshared Key
                                     </label>
-                                    <input type="text" class="form-control" id="client_preshared_key" name="client_preshared_key" placeholder="Autogenerated">
+                                    <input type="text" class="form-control" id="client_preshared_key" name="client_preshared_key" placeholder="Autogenerated - enter &quot;-&quot; to skip generation">
                                 </div>
                             </details>
                         </div>