Fix link issues and text revision type

This ensures that link targets do not get overwritten by empty ID
properties on RevisionLink creation, that embedded links get rendered
properly instead of overwritten and that space links point to a space
rather than a partially completed tiddler shortlink
Refactor tiddler creation

This adds a revision builder for building new revisions (which is an
involved process).

This is in anticipation of adding support for extraclusion, the addition
of which shouldn't be handled inside the Tiddler model (extraclusion
involving adding unrelated tiddlers of course).