renamed some classes and Twig functions to more descriptive names (refs #6871)

HttpContentRenderer has been renamed to FragmentHandler.
The RendererStrategy subnamespace has been renamed to Fragment.
The strategy classes now have Fragment in their names.
ProxyRouterListener has been renamed to FragmentListener
The router_proxy configuration entry has been renamed to fragments.
Merge branch '2.2'

* 2.2:
  fixed regression in the Finder component (it was possible to use it without using exec before, closes #6357)
  fixed a circular call (closes #6864)
  typo
  [Security] [Tests] added unit tests for the UserPasswordValidator class and made the validator service for the UserPassword constraint configurable.
  fixed wrong indentation
  tweaked previous commit
  [HttpKernel] Fix the URI signer (closes #6801)
  Add Arabic translations.
  [HttpKernel] fixed regression when rendering an inline controller and passing some objects  (closes #6822)
  [FrameworkBundle] fixed typo
  renamed some classes and Twig functions to more descriptive names (refs #6871)
  Classcollectionloader: fix traits + enhancements
  Fix a deprecated method call in the tests
  Update `composer.json` files: - to allow versions ~2.2 (>=2.2,<3.0) of Doctrine DBAL, ORM & Common - fixed Propel1 versions difference between main and bridge files - fixed Twig versions difference between main and bridge files - to allow versions ~1.11 (>=1.11,<2.0) of Twig - fixed Locale ext-intl version to accept all, not non-existing version
  Correct comment in NativeSessionStorage regarding session.save_handler
  [Security] Add PHPDoc to AuthenticationEvents