WelcomeCronk: Fix default cronk setting

Default cronk is defined in userpreferences.xml:

<ae:parameter name="org.icinga.cronk.default">portalHello</ae:parameter>

If you use an other cronk which uses parameters the old method did not
respect params sections. The new way (which is implemented in
CronkListingPanel.js) loads all cranks from remote (also hidden cronks)
and create a valid record (xml and database cronk)