Disable admin by default

[covex-nn]

I am targeting this branch, because this is BC break

Changelog

### Changed
- Disable admin configuration by default

Subject

I believe, that disabling admin in configuration is more correct, because it can be enabled only when sonata.notification.backend.doctrine was enabled.

A default configuration, i.e. recipe for sonata-project/notification-bundle could be like this:

sonata_notification:
    consumers:
        register_default: false
    admin:
        enabled: false

Also, default configuration could contain commented configuration for backend: sonata.notification.backend.rabbitmq. Anyway, i have already created PR with recipe for this bundle in febuary symfony/recipes-contrib#302

Additional configuration, i.e. a recipe for a new package sonata-project/notification-orm-pack could be like this (i created a gist with entity class):

sonata_notification:
    backend: sonata.notification.backend.doctrine
    backends:
        doctrine: ~
    class:
        message: App\Entity\SonataNotificationMessage
    admin:
        enabled: true

So, additional configuration will override default.

[kunicmarko20]

This is a BC break? You can't do it on 3.x

[kunicmarko20]

cc @sonata-project/contributors

[kunicmarko20]

I would rather add deprecation now, change it on master and for now add it to recipe

[covex-nn]

No! It doesn't matter, i will update right now

[covex-nn]

I changed base branch of this PR

[kunicmarko20]

Please add upgrade note. Should we deprecate this on stable @sonata-project/contributors ?

[greg0ire]

What should probably be deprecated on stable would be not specifying this boolean (and expecting it to default to true)

[covex-nn]

May be if class sonata_notification.class.message is exists - admin should be enabled automaticaly? And may be this parameter cannot be enabled my configuration - it can only be disabled? Can it be done for current stable?

And i do not know how to deprecate configuration )

[covex-nn]

Since #326, even if admin is enabled, services for SonataAdmin are added only if notification backend is sonata.notification.backend.doctrine. I guess this PR can be closed, wdyt? Anyway, i've added upgrade note, it this PR will be accepted - it is ok too! =) Also PR in symfony/recipes-contrib should be updated (admin can be enabled all the time after new future release of NotificationBundle)

[core23]

Thanks @covex-nn