Support Space Closure Default for Wiki Link Macro

The Wiki Link Macro allows to render a link to a projectdoc document on the delegate space or the search space. This is controlled by a parameter of the macro.

For index spaces this property should be set to 'search space' by default, whereas it should default to 'delegate space' on all other kinds of spaces (we currently know of).

Therefore we need to default to the space closure – being either search-space or delegate space (default) – which is a space property with name 'default-space-closure'.