Add admin extension to edit base fields of base block document

Pull request for issue #28 make cache related meta data editable in the admin

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | #28 
| License       | MIT
| Doc PR        |  https://github.com/symfony-cmf/symfony-cmf-docs/pull/172