[#1838] Set default FTS language using ckan.datastore.default_fts_lang

If you set `ckan.datastore.default_fts_lang` on the INI file, it'll be used as
the default language to be used when creating full-text search indexes and
queries. It can be overwritten on a per-request basis by using the `lang`
parameter. If none exist (neither `lang` or `ckan.datastore.default_fts_lang`),
"english" is used as a default.