[BUGFIX] backendUrl may already have a querystring
[BUGFIX] backendUrl may already have a querystring

data-backend-url might already contain a query string. Hence it is wrong to always append ?url.
This patch handles the situation properly and adjusts to &url if necessary.
ArneBab/shariff,rolandgeider/shariff,andrelohmann/shariff,andrelohmann/shariff,ArneBab/shariff,bozana/shariff,craiq/shariff,ArneBab/shariff,P1tt187/shariff,kraftner/shariff,medienverbinder/shariff,craiq/shariff,kraftner/shariff,bozana/shariff,craiq/shariff,medienverbinder/shariff,heiseonline/shariff,P1tt187/shariff,rolandgeider/shariff,medienverbinder/shariff,andrelohmann/shariff,rolandgeider/shariff,bozana/shariff,P1tt187/shariff,kraftner/shariff