'yii\base\Security' component created.
Component 'security' added tp the base application
'Security' helper usage switched to 'security' application component.
Docs regarding `Security` component usage updated.
Security helper files removed
Notes about `Security` class refactoring added to CHANGELOG.md and UPGRADE.md
Doc comments at `yii\base\Security` fixed
Extra namespace at docs removed
Upgrade not about `Security` component adjusted
Method `Security::compareString()` extracted
Fallback for `Security::generateRandomKey()` added
Option `Security::deriveKeyStrategy` added
Option `Security::passwordHashStrategy` added
Doc comments at `Security` updated
Option `Security::useDeriveKeyUniqueSalt` added
Upgrade note about `Security` updated
Option `Security::autoGenerateSecretKey` added
Fallback at `Security::generateRandomKey()` removed
Doc comments at `Security::generateRandomKey()` adjusted
Merge pull request #4089 from klimov-paul/87-security-component-3

Fix #87: security helper converted into component and improved