Use the long form of java.net.URI constructor that quotes the path
component of a URI string containing reserved characters (in the sense
of RFC 2396).

This allows a user to specify an external link with the [[...]] syntax
that contains a space character (for example).

https://bugs.corefiling.com/show_bug.cgi?id=13502
Take the horrible URL class out of the normal code path, and make it
clear why it is used at all.
Merge branch 'bug13502', to fix handling of wiki links with spaces in them.