- 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.

https://getcomposer.org/doc/03-cli.md#require

`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.

Minor consistency issue. See the following files for more details:
1. framework/views/migration.php
2. framework/console/controllers/MigrateController.php

```
interface IdentityInterface
{
    /**
     * Finds an identity by the given ID.
     * @param string|integer $id the ID to be looked for
     * @return IdentityInterface the identity object that matches the given ID.
     * Null should be returned if such an identity cannot be found
     * or the identity is not in an active state (disabled, deleted, etc.)
     */
    public static function findIdentity($id);
```

Fixes #4147

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

Fixes #3979: Alert widget in advanced app is now able to display multiple flash messages of the same type

Fixes #3358: Removed automatic CSRF meta tag generation by `View`. Added `Html::csrfMetaTags()` and its call to main layout files