Fix warnings, resolve missing layouts, remove deprecated content

The updates in releases [1.6.0](https://github.com/github/training-kit/releases/tag/V1.6.0), [1.6.1](https://github.com/github/training-kit/releases/tag/1.6.1), and [1.7.0](https://github.com/github/training-kit/releases/tag/1.7.0) deprecated a number of layouts. This results in warning output when starting up Jekyll due to missing content.

This PR resolves that be cleaning out the deprecated class chapter-aggregation pages altogether and adding generic "empty" layouts where defaults are expected.

The warning output (shown) below will no longer be present once this merges.

![kit-errors](https://cloud.githubusercontent.com/assets/352082/5583032/e168c95a-902f-11e4-8282-19b70e64f7c3.gif)