Added `yii\composer\Installer::postCreateProject()` and modified the syntax of calling installer methods in composer.json [skip ci]

Used a single database and a single CLI entry point for all test types. Added faker to basic app dev requirements.

Exposed basic app entry URL via codeception.yml, refactored configs of advanced app to read from config and define entry script in a single place

- 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.

`yii\web\Request::cookieValidationKey` is now automatically generated by the installation script for the basic and advanced application templates

- Removed hacks since these are no longer required: https://github.com/Codeception/Codeception/commit/71db233985531f298264dc4ca0160d2aa1c83b7f
- Removed unused helper classes
- Added 2.0 style "actor" to config

Thanks to @Ragazzo and @DavertMik for helping with it.

Fixes #4147

Fixes #4071: `mail` component renamed to `mailer`, `yii\log\EmailTarget::$mail` renamed to `yii\log\EmailTarget::$mailer`