[#1725] Forgot to import ckan.plugins
[#1725] Custom filters should be added inside parenthesis

The extensions can get a single filter (e.g. "age_not_between") and create two
WHERE clauses (e.g. "age < %s OR age > %s"). To avoid hard-to-track bugs
related to the operator precedence in Postgres, we add every custom filter
inside parenthesis.