Add OWL ontology for Tags

Also add and export 'instantiate-item-types
Uncomment tags.lisp from ox.asd
Revamp tags.lisp
Update the exported symbols
Add the skeleton of a tags test-suite
Move ox test files into their own subdir
store-tag now returns the UUID of the new tag
Start moving the tags tests to a separate file
Replace use of get-tag-by-title with get-tag-uuid-by-title
Refactor error-handling in 'store-tag
Complete the tags test-suite