Register built-in models in a standalone file

Move the code registering built-in models to a new file
`lib/builtin-models.js`.
Dismantle `lib/models`.

 - Move core models `Model` and `PersistedModel` to `lib/`.
 - Move `AccessContext` class to `lib/`, since it is not a model.
 - Move all other built-in models to `common/models`.

This is a preparation for extracting model definitions to JSON files.
By splitting the change into multiple commits, git is able to keep track
of file moves (renames).
Auto-load and register built-in `Checkpoint` model
test: remove infinite timeout

The infinite timeout was useful when debugging, which is not a good
reason for keeping it around when not debugging.
test: verify exported models
test: run more tests in the browser

Add two more test files to `test/karma.conf.js`:

 - test/loopback.test.js
 - test/model.application.test.js
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.
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