Move Migrating files (Step 7)

Moved 2 files to migrating/:
- additional-details/migrating-from-react-rails.md
- javascript/angular-js-integration-migration.md

Combined with convert-rails-5-api-only-app.md from Step 5 corrections,
migrating/ now has 3 files total.

All files kept original names per Decision 2.

Author: ihabadham