Advertise yaml_editor more strongly in migrate_sandbox

Problem/Motivation

Migrate Sandbox is way nicer with yaml_editor installed. Yaml Editor is not a requirement though. It's advertised on the module page, but it would be great to suggest it in the admin UI to make it more likely a user will notice the recommendation.

Proposed resolution

We should leverage hook_requirements at install and runtime.

Remaining tasks

User interface changes

API changes

Data model changes