Add innodb_large_prefix=1 to MySQL configuration

[mikl]

This is needed for Drupal 7 sites, because the actions table has a primary key on aid,
which is a varchar(255). When using the new utf8mb4 encoding. If large prefix is not
set, this causes an error:

Specified key was too long; max key length is 767 bytes

(Due to 255*4 being 1020 bytes).

See also http://stackoverflow.com/a/36405914/66851

[lmakarov]

According to the comments in D7's default.settings.php there may be more settings necessary to enable utf8mb4 charset support for D7:

innodb_large_prefix=true
innodb_file_format=barracuda
innodb_file_per_table=true

MySQL docs on innodb_large_prefix also state that innodb_large_prefix=true and innodb_file_format=barracuda have to be set together to allow for large indexes. With MySQL 5.7.7 and up these are set by default and innodb_large_prefix is deprecated.

Further googling on innodb_file_format suggests that it was removed in MySQL 8.0.

Please update the PR according:

• add innodb_file_format=barracuda for 5.5 and 5.6 (so all 3 settings mentions at the top are applied)
• provide a comment on the settings alteration
• remove alterations for 5.7 and 5.8 from the PR

[mikl]

Done, thanks for the feedback @lmakarov.