Converter sync table should support utf8mb4

[netweb]

Originally reported here: ​https://bbpress.org/forums/topic/import-from-phpbb-issues-wordpress-database-error/

BBP_Converter::sync_table should create the bbp_converter_translator table with UTF8MB4 supported.

CREATE TABLE wp_bbp_converter_translator (
        meta_id mediumint(8) unsigned not null auto_increment,
        value_type varchar(25) null,
        value_id bigint(20) unsigned not null default '0',
        meta_key varchar(255) null,
        meta_value varchar(255) null,
PRIMARY KEY  (meta_id),
        KEY value_id (value_id),
        KEY meta_join (meta_key, meta_value) )
DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;

Related: ​#WP35958 Failing dbDelta tests due to MySQL error
#1071 - Specified key was too long; max key length is 1000 bytes

Quoting ​jdgrimes:

I think that this is partly an issue with the tests, since a key on a VARCHAR(255) field can only be 191 in length, when using utf8mb4. So I think that the keys ought to look like this:

	KEY key_1 (column_1(191)),
	KEY compound_key (id,column_1(171)),

[netweb]

Another case of this popping up, reported ​here

[johnjamesjacoby]

See also #3026.

Fixed in r6129.

[johnjamesjacoby]

Assigning all closed & unassigned tickets in the 2.6 milestone to myself.