Convert both data and database tables to UTF-8 character set.

• Requires the admin_forum permission.
• Uses the convert_utf8 sub template of the Admin template.
• Only works if UTF-8 is not the global character set.
• Supports all character sets used by SMF's language files.
• Redirects to ?action=maintain after finishing.
• Is linked from the maintenance screen (if applicable).
• Accessed by ?action=convertutf8.