fix #19 by adding support for clone

A user may choose to clone when:

* they don't have permission to write the tiddler
* when the tiddler is from a different space (even if they could
  write it if they were in the home space of the tiddler)

The general mechanism is to:

* add a clone button
* change the bag on the tiddler to the current space's public bag
* start up a regular edit