feat: add even more information to definitions

### Additions

* added new field `prerequisites: [String!]!` to `DefinitionType` type
* added new field `targetAudience: String!` to `DefinitionType` type
* added new field `subject: String!` to `EmailTemplateType` type
* added new field `order: Int!` to `LearningObjectiveType` type

Closes #357