Added alternative method to multi-value field migration as comment
added migration for field_lesson_last_peer_review
added migration for field_lesson_overview
restored the first method of multi-value  field migration (the one which migrates only 1 value instead of all multiple values)
added migration for field_lesson_steps
added migration for field_lesson_project_type
added migration for field_lesson_type
rearranged the code (no new code added) of the fields to be migrated, so that similar fields are one after other
added migration for field_lesson_prerequisites
Added migration for Ladder Overview: field_lesson_overview_value (reused field in ladder content type)