Add "magic link" functionality

When using a device that is bothersome to log in on (e.g. a Kindle) you can use a magic link to log in via another device.

Configuration was added and is disabled by default.

cps/templates/admin.html

@@ -64,6 +64,7 @@
         <th>{{_('Uploading')}}</th>
         <th>{{_('Public registration')}}</th>
         <th>{{_('Anonymous browsing')}}</th>
+        <th>{{_('Remote Login')}}</th>
     </tr>
       <tr>
         <td>{{config.config_calibre_dir}}</td>
@@ -73,6 +74,7 @@
         <td>{% if config.config_uploading %}<span class="glyphicon glyphicon-ok"></span>{% else %}<span class="glyphicon glyphicon-remove"></span>{% endif %}</td>
         <td>{% if config.config_public_reg %}<span class="glyphicon glyphicon-ok"></span>{% else %}<span class="glyphicon glyphicon-remove"></span>{% endif %}</td>
         <td>{% if config.config_anonbrowse %}<span class="glyphicon glyphicon-ok"></span>{% else %}<span class="glyphicon glyphicon-remove"></span>{% endif %}</td>
+        <td>{% if config.config_remote_login %}<span class="glyphicon glyphicon-ok"></span>{% else %}<span class="glyphicon glyphicon-remove"></span>{% endif %}</td>
   </table>
   <div class="btn btn-default"><a href="{{url_for('configuration')}}">{{_('Configuration')}}</a></div>
   <h2>{{_('Administration')}}</h2>

cps/templates/config_edit.html

@@ -93,6 +93,10 @@
         <input type="checkbox" id="config_public_reg" name="config_public_reg" {% if content.config_public_reg %}checked{% endif %}>
         <label for="config_public_reg">{{_('Enable public registration')}}</label>
     </div>
+    <div class="form-group">
+      <input type="checkbox" id="config_remote_login" name="config_remote_login" {% if content.config_remote_login %}checked{% endif %}>
+      <label for="config_remote_login">{{_('Enable remote login ("magic link")')}}</label>
+    </div>
     <h2>{{_('Default Settings for new users')}}</h2>
     <div class="form-group">
       <input type="checkbox" name="admin_role" id="admin_role" {% if content.role_admin() %}checked{% endif %}>

cps/templates/login.html

@@ -3,6 +3,7 @@
 <div class="well col-sm-6 col-sm-offset-2">
   <h2 style="margin-top: 0">{{_('Login')}}</h2>
   <form method="POST" role="form">
+    <input type="hidden" name="next" value="{{next_url}}">
     <div class="form-group">
       <label for="username">{{_('Username')}}</label>
       <input type="text" class="form-control" id="username" name="username" placeholder="{{_('Username')}}">
@@ -17,6 +18,9 @@
       </label>
     </div>
     <button type="submit" name="submit" class="btn btn-default">{{_('Submit')}}</button>
+    {% if remote_login %}
+    <a href="{{url_for('remote_login')}}" class="pull-right">{{_('Log in with magic link')}}</a>
+    {% endif %}
   </form>
 </div>
   {% if error %}

cps/templates/remote_login.html

@@ -0,0 +1,40 @@
+{% extends "layout.html" %}
+{% block body %}
+<div class="well">
+  <h2 style="margin-top: 0">{{_('Remote Login')}}</h2>
+  <p>
+    {{_('Using your another device, visit')}} <a href="{{verify_url}}">{{verify_url}}</a> {{_('and log in')}}.
+  </p>
+  <p>
+    {{_('Once you do so, you will automatically get logged in on this device.')}}
+  </p>
+  <p>
+    {{_('The link will expire after %s minutes.' % 10)}}
+  </p>
+</div>
+{% endblock %}
+
+{% block js %}
+<script type="text/javascript">
+  (function () {
+    // Poll the server to check if the user has authenticated
+    var t = setInterval(function () {
+      $.post('{{url_for("token_verified")}}', { token: '{{token}}' })
+        .done(function(response) {
+          if (response.status === 'success') {
+            // Wait a tick so cookies are updated
+            setTimeout(function () {
+              window.location.href = '{{url_for("index")}}';
+            }, 0);
+          }
+        })
+        .fail(function (xhr) {
+          clearInterval(t);
+
+          var response = JSON.parse(xhr.responseText);
+          alert(response.message);
+        });
+    }, 5000);
+  })()
+</script>
+{% endblock %}