Fix Event Url and Url Display

Hello,

i noticed that if i edit one event with url set, it shows the "new" event url display, 

if i edit another event without url, the form continues to show the url display from the other event,

if i try to create a new event, the same problem occurs.

and as a bonus i do not update the event url to empty if open event doesn't has url set,
to uniformize with a new event that shows 'http://' as default value.

Thanks in advance