VCS: add support for phrase matching in PostgreSQL fulltext search

For PostgreSQL, where this feature is not supported natively, we will match the phrase "foo bar" using:

to_tsquery('foo & bar') @@ to_tsvector(fulltext)
  AND fulltext LIKE '% foo bar %'

This means that the fulltext column will have to be in clear text, pre-canonicalized (lowercase, no diacritics), and with an initial and trailing space.

Note that a more complex match like

abc "foo bar" -"gee man"

will have to be turned into something like:

to_tsquery('abc & foo & bar') @@ to_tsvector(fulltext)
  AND (fulltext LIKE '% foo bar %' AND NOT fulltext LIKE '% gee man %')

i.e., the initial fulltext match has to match a superset of the complete query, so it cannot easily contain excluded phrases.

Algorithm:

1. pre-filter using fulltext on a query that is a superset of the original query (remove negative phrases (which are at AND level), and split positive phrases into ANDed words),
2. AND with a LIKE-based search for all terms, except those that are already exactly matched by the first part, i.e., toplevel ANDed non-phrases.