Fix mistakes in categories files

This was causing some dataset pages to crash
Add "purpose" to schema
Return single value for convert to tags fields

Return a single value (rather than a list with one item in it) for tag
vocabulary / convert_to_tags fields.
More schema tweaks

These got the post-and-validate paster command to pass.
Some dataset read template fixes

(Now that we're returning single values instead of lists for tag vocab
fields.)
Fix some invalid HTML in the package form
Fix purpose field in package form
Merge branch 'master' of github.com:okfn/ckanext-sgdata