[FrameworkBundle] Add shortcut methods to controllers

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11166
| License       | MIT
| Doc PR        | 

To-do list : 
- [x] submit changes to the documentation

Added redirectToRoute, addFlash, isGranted and checkGranted to controllers. The code seems so simple I didn't feel like adding controller tests was needed since we're just shortcuting other services calls.
Isn't this too optimistic given that [SessionInterface](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/HttpFoundation/Session/SessionInterface.php) doesn't enforce ``getFlashBag()``?