- Fix names and references guides internals.
- Add internals Spanish translation.
- Drafting corrections Spanish guides.

issue #4719

- Moved everything test-related into `tests` directory. Codeception tests are in `codeception`.
- Removed database module since we're using fixtures and migrations.
- Moved console entry points and bootstrap into `tests/codeception/bin`.
- Adjusted travis build scripts.
- Adjusted documentation to be consistent and reflect changes made.

Fixes #4324: removed old way of developing Yii from the docs since app/link is much better/simpler workflow

PS: should `start-looking-head.md` be `start-looking-ahead.md`? I remembered that is called look ahead.