Fixes a bug in Migrate messages UI. Solution found in the following post: https://www.drupal.org/node/1794236#comment-6724556. As mentioned by mikeryan, patch #9 applied manually.
Feat. Catalog updated: Subqueries removed in ContributorMigration and ArticleContributorsMigration classes.
Feat. Catalog updated: Mistral migration settings admin page added.
Feat. Catalog updated: ArticleMigration class altered to replace hard-coded migration variables (url, default cover image) by variable values set up in Mistral migration setting page.