Refactor URLWrapper subclasses to make slugs settable again (#2242)
Refactor URLWrapper subclasses to make slugs settable again (#2242)

URLWrapper slugs are intended to be settable, but because all of the
concrete URLWrapper subclasses override the _getter_ for .slug using
@property, the setter in the superclass becomes inaccessible.  Fix
this by refactoring out the code that the subclasses actually need
to override to a normal helper method.

zackw/pelican,zackw/pelican,zackw/pelican