Save and load page ids in and from link mappings

Some (absolute) links from confluence only contains the page id of the document. Supporting page ids in the link mapping feature will allow convert more links successfully.

Page id to document reference mappings will be stored in a "[spaceKey]:ids" key (with "[spaceKey]" replaced with the actual space key). ":" has been chosen arbitrarily as a character allowed in XWiki document names, but not in Confluence space keys.We found some use of "/" in Confluence space keys we haven't sorted out, so we avoided using this character.
The input property will accept such keys too, but will not require that ids stored by spaces and will also accept that all page ids are stored in a single ":ids".