indexing getLastCommentUrl absolute_url is absolute wrong

[jensens]

see https://github.com/collective/Products.Ploneboard/blob/master/src/Products/Ploneboard/content/PloneboardConversation.py#L204

a) why is this an index? metadata should be enough
b) why is it absolute? this will clash with http/https environments!!! also indexing from an edit/internal url does not work.

needs fix