Support search using regex

[jtojnar]

When user searches a term starting and ending with a forward slash, the items will be matched against the regular expression between the slashes.

The syntax of the regular expressions is database-engine specific:

• MySQL uses POSIX extended https://dev.mysql.com/doc/refman/5.7/en/regexp.html
• PostgreSQL uses POSIX extended with some extensions https://www.sqlite.org/lang_expr.html#the_like_glob_regexp_and_match_operators
• SQLite uses PHP’s PCRE https://www.postgresql.org/docs/12/functions-matching.html#FUNCTIONS-POSIX-REGEXP, https://www.php.net/manual/en/book.pcre.php

---

The filtering works but the interface is still incomplete. In particular, the matches will not be highlighted in the articles, and the search regex will be shown split on spaces. Fixed.

[jtojnar]

I want to base this onto #1216, which will make it easier to distinguish the search terms in the UI.