Add NOT_EQUAL operator for StringOperatorType

[VincentLanglet]

Subject

I am targeting this branch, because BC. This is a new feature.
There is contains and not contains operator, but only the equal operator.

Changelog

### Added
- `NOT_EQUAL` operator for `StringOperatorType`

[peter-gribanov]

Maybe you should add localization?

[VincentLanglet]

Maybe you should add localization?

The key already exists, from the equalOperatorType

[peter-gribanov]

The key already exists, from the equalOperatorType

Sorry. I didn't notice.

[phansys]

IMO, we should add an upgrade note with this change.

[VincentLanglet]

IMO, we should add an upgrade note with this change.

With what kind of message ?

[phansys]

With what kind of message ?

I think something like this could work:

Added "Not equal" in the default list for "choices" option at Sonata\AdminBundle\Form\Type\Operator\StringOperatorType, in order to allow filtering by strings that are not equal to the model data.

[VincentLanglet]

With what kind of message ?

I think something like this could work:

Added "Not equal" in the default list for "choices" option at Sonata\AdminBundle\Form\Type\Operator\StringOperatorType, in order to allow filtering by strings that are not equal to the model data.

Then I think the same as sonata-project/SonataDoctrineORMAdminBundle#1196 (comment)
This seems enough in Changelog to me.

New features are not added to the Upgrade-x files.
Generally it's more deprecation, because the user has to do something to avoid them.

[phansys]

New features are not added to the Upgrade-x files.

I agree with this premise, in cases where the new feature does not affect the previous behavior without an explicit user's intervention (by adding a new argument, a method call, by setting some service configuration, etc.). Even if a new choice in a dropdown could be considered a slight change, I think it is changing the previous behavior (it's adding a new option in an existing dropdown and allowing end users to filter using a new operator).
Obviously, we could set our "threshold" between what deserves an upgrade note and what doesn't.