Mount REST at app.restApiRoot + remove explorer basePath [DO NOT MERGE YET]

- Mount REST API at app.get('restApiRoot')
- Remove `basePath` option from loopback-explorer initialisation, as the plugin already uses `app.get('restApiRoot')` by default.

Requires strongloop/loopback-explorer#10.

We need to bump up the minimum loopback-explorer version in template/package.js before merging this change.

/to: @ritch please review.