Add an option to specify a webhook URL for updates in the downloads section

and tweak the help texts for the original source web hook a bit. Also remove
the superfluous inline help code from the SourceConf form that was actually
not used.

This feature was sponsored by Scilab.

src/IDF/Form/SourceConf.php

@@ -49,13 +49,10 @@ class IDF_Form_SourceConf extends Pluf_Form
                           'widget' => 'Pluf_Form_Widget_PasswordInput',
                           ));
         }
-        Pluf::loadFunction('Pluf_HTTP_URL_urlForView');
-        $url = Pluf_HTTP_URL_urlForView('idf_faq').'#webhooks';
         $this->fields['webhook_url'] = new Pluf_Form_Field_Url(
                     array('required' => false,
                           'label' => __('Webhook URL'),
                           'initial' => $this->conf->getVal('webhook_url', ''),
-                          'help_text' => sprintf(__('Learn more about the <a href="%s">post-commit web hooks</a>.'), $url),
                           'widget_attrs' => array('size' => 35),
                           ));