Fixes #2629: `Module::controllerPath` is now read only, and all controller classes must be namespaced under `Module::controllerNamespace`

Fixes #2653: Fixed the bug that unsetting an unpopulated AR relation would trigger exception (qiangxue)

Fixes #1535: Improved `yii\web\User` to start session only when needed. Also prepared it for use without session.

Fixes #1564: Removed `yii\web\Session::autoStart` and added `hasSessionId`. Session will be automatically started when accessing session data

for prev, next, first and last page.
http://www.w3.org/TR/html401/struct/links.html#h-12.1.2

Changed `Controller::afterAction()`, `Module::afterAction()` and `ActionFilter::afterAction()` to pass `$result` by value instead of reference

Fixes #2544: Changed `DetailView`'s `name:format:label` to `attribute:format:label` to match `GridView`

Fixes #2527: Source language for `app` message category was always `en` no matter which application `sourceLanguage` was used

Fixes #2392: Added `addCssStyle()`, `removeCssStyle()`, `cssStyleFromArray()` and `cssStyleToArray()` to `Html`

fixes #2519: MessageSource removed translation messages when event handler
was bound to `missingTranslation`-event

Fixes #2491: Added support for using the same base class name of search model and data model in Gii [skip ci]

allow extending only one class to add scopes, fixes #2146

TODO:

- [ ] adjust guide docs
- [ ] adjust README files of extensions
- [ ] finish work and fix test breaks

Fixes #2360: Added `AttributeBehavior` and `BlameableBehavior`, and renamed `AutoTimestamp` to `TimestampBehavior`

Fixes #2490: `yii\db\Query::count()` and other query scalar methods now properly handle queries with GROUP BY clause