Since we are removing all the MP2 migration code the table
wp_mailpoet_mapping_to_external_entities is not needed anymore. It was
used to map MP2 objects to MP3 objects. This commit removes the code
that creates this table for new installs and deprecates its model.
[MAILPOET-4376]
MySQL has maximum key lengths for indexes, and the number can vary based
on storage engine and MySQL version. Using utf8mb4, the index would be
255 * 4 = 1020 bytes long, exceeding the MyISAM maximum of 1000 bytes
and the MySQL 5.6 maximum of 767 bytes.
See Column Prefix Key Parts in the MySQL reference:
https://dev.mysql.com/doc/refman/8.0/en/create-index.html
By reducing this length to 191, we ensure that the maximum likely index
length will be less than the 767 lower bound (191 * 4 = 764).
MAILPOET-4489
This commit fixes the logic of the code that enables third party
libraries by default. It was meant to enable those libraries only for
existing installs when the code was introduced in cfae753601
to avoid breaking functionality. But due to a bug, it was also enabling
those libraries for new installs.
To fix this issue, this commit moves the code to the Migrator class and
makes sure it is only executed for existing installs and only once when
doing a version update.
[MAILPOET-4285]
