made twig extension service private
merged branch kriswallsmith/master (PR #6842)

This PR was merged into the master branch.

Commits
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731cd49 made twig extension service private

Discussion
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[TwigBundle] made extension a private service

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~
[HttpKernel] made the Request required when using rendering strategies

The previous code allowed to pass null as a Request but that does not
really make sense as rendering a sub-request can only happen from a
master request. This was done to ease testing but that was a mistake.
removed the need for a proxy route for rendering strategies
made the proxy path configurable
[HttpKernel] renamed path to _path to avoid collision
renamed proxy to router_proxy