Multiple values in URL not split by whitespace

[jeroen]

The Description URL field is allowed to have multiple URLs split only by a space. It is even allowed to have non urls in there (comments).

Doing so leads to an invalid \url{} in the new "Useful links:" section because roxygen seems to treat the entire field as the URL.

The solution is probably to grep for actual URL strings.

[maelle]

#1420