Search query string rework

[mgdigital]

• search_string field is removed
• text search vector for content and torrent_contents is generated in code with improvements in normalisation and tokenisation
• the slow LIKE query is removed from text search
• a query DSL is implemented which translates the input search string to a Postgres tsquery
• a CLI command (reindex) is provided for updating the tsv for pre-existing records
• the unused tsv and search_string fields are removed from the torrents table

See #89