This PR was merged into the 2.1.x-dev branch.

Discussion
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Added WebLink component integration

I made the necessary changes to support the new WebLink component:

- `HttpKernelServiceProvider` registers the `AddLinkHeaderListener` if the component is available
- `TwigServiceProvider` adds the `WebLinkExtension` if the component and the Twig bridge are available
- Both changes are tested
- The documentation has been amended (the link to the Symfony website doesn't work as 3.3 hasn't been released yet but I believe it's the good URL)

Commits
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37bd5aed Added WebLink component integration