fixes #1821

Fixes #1835: `CheckboxColumn` now renders checkboxes whose values are the corresponding data key values

Fixes #1827: Debugger toolbar is loaded twice if an action is calling `run()` to execute another action

Changed Yii autoloader to support loading PSR-4 classes only (i.e. PEAR-styled classes not supported anymore)

Doc fixes

Typo fix

Fix doc for generateSalt.

$cost must be in range 4-31.

verify #1755 works

Conflicts:
	framework/yii/db/ActiveQuery.php

Update view.md

Fixed URL parsing so it's now properly giving 404 for URLs like `http://example.com//////site/about`

1. It results in much smaller download and takes less disk space (25MB instead of 180MB for yii-basic + dependencies).
2. It requires less utilities to be installed (no VCS binaries required).
3. It takes less time to install dependencies.

Add handler property to yii\web\Session

and 7ba6a063

Fixed label attributes in BaseHtml