Make SonataAdminBundle dependency optional

[supersmile2009]

Subject

This PR makes SonataAdminBundle dependency optional.

I am targeting 5.x branch, because v5 hasn't got to a final release yet and BC breaks can potentially be still accepted and there were no objections to that in the referenced issue.

Closes #1499

Changelog

### Changed
- SonataAdminBundle dependency is now optional. Trying to access `sonata_user.userAdmin` variable in Twig templates will throw a `\LogicException` if SonataAdminBundle is not installed.

To do

• Update the documentation;
• Update the tests;
• Add an upgrade note.

Notes:

1. Do you think nullable admin Pool in GlobalVariables is OK or it would be better to conditionally register pool as another Twig variable (e. g. sonata_user_admin)?
2. I've checked the Media and Classification bundles (referenced in the linked issue as bundles with optional Admin dependency) and there seems to be no full-blown integration tests with running separate test-suits with and without Admin. So I took a modest approach with simply unit-testing the extension with a different config.

[supersmile2009]

Failing tests are not caused by the changes in this PR. See #1504

[supersmile2009]

Disregard my last comment. It appears that some Symfony FrameworkBundle configs are cleverly applied depending on whether a certain dependency is expected to be available in non-dev install. I had to add framework.assets config to force activate AssetExtension for twig. Otherwise FrameworkBundle de-registers the service, because symfony/asset is not a prod dependency anymore (unlike it was before as a dependency of the admin bundle)

[jordisala1991]

Thanks @supersmile2009 !