Improvements in exhibition of the changed files in Gii.

Added `yii\base\Module::getParam($name, $defaultValue = null)`  to conveniently work with application parameters

Moved most of the user-related logic into form models

fixed i18n link

Adding support for the X-HTTP-Method-Override header

This reverts commit 80be824f.

This reverts commit b82a9b1c.

Fixes #1812: Hide potential warning message due to race condition occurring to `Session::regenerateID()` call

Fix url for db panel.

Fixture controller global fixtures option

Faker controller improved

Fixes #2298: Fixed the bug that Gii controller generator did not allow digit in the controller ID (qiangxue)

Fixes #2303: Fixed the bug that `yii\base\Theme::pathMap` did not support dynamic update with path aliases