MYSQLi bug - Incorrect string value: '\xF0\x9F\x8E\x89\xF0\x9F...' for column `phpbb`.`phpbb_privmsgs`.`message_subject`

Related to CUSTDB-826 and (tangentially) to PHPBB3-16981.

When sending a PM with the subject line "Re: Thanks! <img src=abc onerror=alert(1)> 🎉🎉🎉 &amp;quot; &amp;" on my local install, the following error is displayed:

SQL ERROR [ mysqli ]

Incorrect string value: '\xF0\x9F\x8E\x89\xF0\x9F...' for column `phpbb`.`phpbb_privmsgs`.`message_subject` at row 1 [1366]

An SQL error occurred while fetching this page. Please contact the Board Administrator if this problem persists.

This is despite using the correct `utf8mb4` encoding for the version of MySQL I'm using.

The error indicates it comes from mysqli, and swapping out `sql_escape` in `phpbb/db/driver/mysqli.php` so that it uses `utf8_encode_ucr` appears to fix the problem:

    function sql_escape($msg)
    {
        return @mysqli_real_escape_string(
            $this->db_connect_id,
            utf8_encode_ucr($msg)
        );
{{    }}}

I don't see any drawbacks to doing this either, as `utf8_encode_ucr` is idempotent for any string that doesn't contain unencoded non-BMP characters, so double-encoding won't be an issue.

For reasons I mentioned in the comments to CUSTDB-826, per Matthias Bynens PBUH, the ideal solution would be to deprecate/remove `utf8_encode_ucr` entirely and simply fix the problem at-source by only ever using `utf8mb4` encoding... however, running
`mysqli_set_charset($this->db_connect_id, 'utf8mb4')` seems to do nothing, and in any case `utf8_encode_ucr` is already used in enough places that removing it now would be risky and probably just unnecessarily complicate things. Meanwhile, https://github.com/phpbb/phpbb/pull/6377 should fix PHPBB3-16981 and any more occurrences of the opposite problem (encoded HTML entities being displayed to the user in contexts where the original character should be shown instead).

PR to follow.