Small fixes for the sake of consistency

Using constants instead of literals in Behavior docs

Mail events before/after

Removed redundant code in detachBehaviors()

After ensureBehaviors() call $this->_behaviors is an array already.
And since detachBehavior() unsets elements in $this->_behaviors, $this->_behaviors turns out to be an empty array after all iterations.

improved database panel in debug module

debug module log panel improved

Typo in changelog [skip ci]

Merge branch 'add_old_features_twig_view_renderer' of github.com:dev-mraj/yii2 into dev-mraj-add_old_features_twig_view_renderer

docs about response