Remove the KoboUserKey-based Authentication.

Instead, the user generates the api_endpoint url to set on their device
by visiting http://.../kobo_auth/generate_auth_token.
The generated url will contain a RemoteAuthorizationToken that will be
included on all subsequent requests from the device to the kobo/
endpoints. (In contrast, the device is authenticated using a session cookie on
requests to the download endpoint).

Also use Flask.url_for to generate download urls.

cps/templates/user_edit.html

@@ -28,10 +28,6 @@
       <input type="email" class="form-control" name="kindle_mail" id="kindle_mail" value="{{ content.kindle_mail if content.kindle_mail != None }}">
     </div>
     <div class="form-group">
-    <div class="form-group">
-        <label for="kobo_user_key">{{_('KoboStore UserKey')}}</label>
-        <input type="password" class="form-control" name="kobo_user_key" id="kobo_user_key" value="" autocomplete="off">
-    </div>
     <label for="locale">{{_('Language')}}</label>
         <select name="locale" id="locale" class="form-control">
             {%  for translation in translations %}