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.

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

This reverts commit 2bb6f135.