models: move User LDL def into `user.json`
models: move Email LDL def into `email.json`
models: move Application LDL def into a json file

Move some of the comments describing properties into jsdoc.
bultin-models: code clean-up

Extract `createModel(definitionJson, customizeFn)` to reduce
code duplication.
models: move AccessToken LDL def into a json file
models: move Scope def into its own files
models: move ACL LDL def into a json file
models: move RoleMapping def into its own files
models: move Role LDL def into a json file
models: move Checkpoint LDL def into a json file
models: move Change LDL def into a json file