Prepare the application for Symfony 3

[javiereguiluz]

We don't need to update the Symfony Demo app as soon as Symfony 3 is released, but we need to prepare the application anyway.

TODO list

• Don't use the deprecated "_self" variable in Twig templates (Don't use the deprecated "_self" variable in Twig templates #242)
• Remove YAML configuration files deprecations (Prepare the YAML configuration files for Symfony 3 #246)
• Fix the Form component deprecations for Symfony 3 (Fix the Form component deprecations for Symfony 3 #250)
• Fix the deprecations related to ResourceInterface::isFresh() (Fix the deprecations related to ResourceInterface::isFresh() #251)
• Fix the CSRF token generator deprecation (Use csrf_token_generator instead of deprecated csrf_provider #274)
• Update the use of _self in Twig templates (In Twig 2.x, _self is a string instead of a Template instance #322)
• ...

[jzawadzki]

I would add moving to symfony3 directory structure?

[ogizanagi]

See #274 about another issue we'll encounter in 3.0.

[wouterj]

The first item can be removed, _self is no longer deprecated in Twig. (this didn't have anything to do with Symfony 3 btw)

[wouterj]

Item 3 is fixed by #253

[wouterj]

Item 4 is related to Assetic and not this app: #251

[Tobion]

Unless some new migration problems come up, this issue can be closed as all those issues above have been solved.

[Tobion]

The lock file needs to be updated for assetic.

[radykal-com]

Hello,
#242 can be marked as done in the checklist as the featured was un-deprecated and the issue itself is closed.

[radykal-com]

Same with #251 the tag 2.8 from assetic bundle from 12 days ago fixed it.

[javiereguiluz]

@radykal-com good catches! Everything on the list is now marked as "fixed",so I'm closing this issue. Thanks!

[voronkovich]

@javiereguiluz, you forgot that since Symfony 3 the _self variable contains a string, not an instance of a template. We must fix it.