feature #13264 URL manipulations as a Twig extension (fabpot)

This PR was merged into the 2.7 branch.

Discussion
----------

URL manipulations as a Twig extension

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | symfony/symfony-docs#4805

While working on the new asset component, I realized that the "absolute URL" feature was misplaced and would benefit from being exposed as a Twig function (composition is always a good thing). Then, I wondered if having a Twig function to generate a relative path (like done by the Routing component would also make sense). And here is the corresponding PR.

```jinja
{# generate an absolute URL for the given absolute path #}
{{ absolute_url('/me.png') }}

{# generate a relative path for the given absolute path (based on the current Request) #}
{{ relative_path('/foo/me.png') }}

{# compose as you see fit #}
{{ absolute_url(asset('me.png')) }}
```

As you can see, we require an absolute path for both functions (and we even add the leading slash if it is omitted), not sure if we want to do otherwise.

ping @Tobion

Commits
-------

0ec852d added a relative_path Twig function
ee27ed8 added an absolute_url() Twig function

DependencyInjection/Compiler/ExtensionPass.php

@@ -42,5 +42,9 @@ public function process(ContainerBuilder $container)
         if ($container->has('fragment.handler')) {
             $container->getDefinition('twig.extension.httpkernel')->addTag('twig.extension');
         }
+
+        if ($container->has('request_stack')) {
+            $container->getDefinition('twig.extension.httpfoundation')->addTag('twig.extension');
+        }
     }
 }

Resources/config/twig.xml

@@ -103,6 +103,9 @@
             <argument type="service" id="fragment.handler" />
         </service>
 
+        <service id="twig.extension.httpfoundation" class="Symfony\Bridge\Twig\Extension\HttpFoundationExtension" public="false">
+            <argument type="service" id="request_stack" />
+        </service>
 
         <service id="twig.extension.form" class="%twig.extension.form.class%" public="false">
             <argument type="service" id="twig.form.renderer" />