Topic title length should check utf-8 encoding

[r-a-y]

In #3189, topic title length is enforced with mb_strlen( $title, '8bit' ). However we should change the encoding from 8bit to utf-8 instead.

To duplicate this problem, try to post a new topic with the following topic title:

“Let’s try to create a new topic!” This should post correctly because it should

This is under 80 characters, but due to the three quotation characters using unicode and due to the current 8bit encoding check, this will not post as the three quotation characters increases the character count by three. Switching the mb_strlen() encoding to utf-8 works.

See attached patch.

[johnjamesjacoby]

In 7397:

Common: Fix incorrect encoding in bbp_is_title_too_long().

This commit changes the way that Forum/Topic/Reply title lengths are calculated, away from 8bit for counting bytes to utf-8 for counting characters.

This change is necessary to allow unicode (multibyte) characters to be more accurately included in the overall length calculation, to better match the intention for this optional limitation.

In trunk, for 2.7.

Props r-a-y.

Fixes #3664.