Course completion success message

[adamwoodnz]

Add a new field on Course post types in the Learn plugin, for entering a custom completion message. This will be displayed on the course completed page instead of the default.

This needs to support being able to add a link. Or it could be 2 separate fields; a completion message field, and link field for linking to a survey.

[kathrynwp]

Just noting that the purpose of this would be to display a specific message for each course, once it's completed. This could also allow us to link to a different survey for different courses – for example, the Community team courses might want a different survey than other courses.