[Security] add an AbstractVoter implementation

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT
| Doc PR        | TODO

The idea is to reduce boilerplate required to create custom Voter, doing most of the work for the developer and guiding him on the path by providing simple requirements via abstract methods that will be called by the AbstractVoter.

P.S. This is meant to be a [DX Initiative](https://github.com/symfony/symfony/issues?labels=DX&state=open) improvement.
Hmm, I'm conflicted on this.
The `VoterInterface` doesn't actually support `$object` to be null: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Security/Core/Authorization/Voter/VoterInterface.php#L52

But then again I know for a fact that it's possible to have non-object voters..

I suppose adding as simple check that object isn't null won't hurt