Codeception test adjustments for basic and advanced applications

RFC 7232 Compliance for yii\filters\HttpCache

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

[skip-ci] Update structure-widgets.md

changed bootstrap namespace to jui

Conflicts:
	.travis.yml

- 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.