Created AbstractSonataAdminExtension - parent class for all SonataDoctrine*Bundle`s + SonataPropelAdmin extension classes

Discussion about this change: sonata-project/SonataDoctrineORMAdminBundle#292

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes (in related bundles will pass after merging this PR to master)
| Fixed tickets | -
| License       | MIT
| Doc PR        | -