Allow emoji in attention titles and descriptions

[ECYaz]

Fixes #371.

Reporting a post or a contribution writes the subject and the reason straight into attention_title and attention_description. Unlike the message fields these never pass through the text formatter, so a four byte character such as an emoji reaches the utf8 columns as-is and MySQL rejects the row:

Incorrect string value: '\xF0\x9F\x86\x95 t...' for column 'attention_title' at row 1 [1366]

This uses utf8_encode_ucr() on the way in, the same thing core does to report text in phpbb\report\controller\report, per @iMattPro's suggestion on #390. Doing it in titania_attention::submit() covers all three places that create attention rows rather than patching each one.

Nothing needs decoding on the way out, since a character reference renders as the character it stands for.

Checked on a local board:

• reporting with an emoji in the subject and in the reason now succeeds
• four byte characters outside the emoji range are handled too, a rare CJK ideograph encodes the same way
• accented characters are left alone, only four byte sequences are touched
• plain text is unchanged, and rows written before this change read back exactly as before
• the encoding is stable, so a caller passing text that already carries a character reference does not end up with it encoded twice
• closing a report does not re-encode it

The columns could instead be moved to utf8mb4, but the driver connects with SET NAMES 'utf8', so the insert still fails at the connection before the column charset applies. That is PHPBB-16137, still awaiting review.