Support URLs with other parameters next to ${size}

The sizeParameter was just appended to the URL after a '?'.
This did not work when the URL itself already contained a '?'.
Now check for existing parameters and insert a '&' instead.

This way, we now support URLs of e.g.:
'/cgi-bin/avatar?uid=${user}&size=${size}'.

Change-Id: I10781a04eeec8bbe68451fab38942a0e305b27f4