allow (sub)worksheets to be worksheet items, and resolve worksheet refer...

Support subworksheets, that is, worksheet items which are worksheets.  This effectively gives the ability to use subdirectories, and reference worksheets "locally" from within a main worksheet.  Unlike directories, there is no hierarchy of worksheets: worksheets simply link to other worksheets.

Substantative changes:
- A new column is added in the database (need migration).
- Worksheet items are now quadruples (bundle, subworksheet, value, type) rather than triples- Changes BundleClient API so that (i) we can resolve worksheets relatively and (ii) get info about them without fetching their items