Replace some references to "entity" term with "model"

[phansys]

Subject

Replace some references to "entity" term with "model" in order to enforce the agnosticism around the model manager.

I am targeting this branch, because these changes respect BC.

Changelog

### Deprecated
- Deprecated `FieldDescriptionInterface::getTargetEntity()` in favor of `FieldDescriptionInterface::getTargetModel()`;
- Deprecated `AdminHelper::getEntityClassName()` in favor of `AdminHelper::getModelClassName()`;
- Deprecated `GenerateObjectAclCommand::getUserEntityClass()` in favor of `GenerateObjectAclCommand::getUserModelClass()`.
- Deprecated `--user_entity` option in favor of `--user_model` at `sonata:admin:generate-object-acl` command.

[VincentLanglet]

@phansys If you want to fully enforce the agnosticism around the model manager, we have to do something about the following lines

SonataAdminBundle/src/Form/Type/AdminType.php

Line 99 in bd257ed

return $associationMapping['fieldName'].'.';

SonataAdminBundle/src/Admin/AdminHelper.php

Line 233 in 4993aeb

$method = sprintf('get%s', $inflector->classify($parentMapping['fieldName']));

SonataAdminBundle/src/Admin/AdminHelper.php

Line 253 in 4993aeb

$method = sprintf('add%s', $inflector->classify($inflector->singularize($mapping['fieldName'])));

We're trying to access to the 'fieldName' key.

If we have a getter to avoid using the key targetEntity, do we need to do similar for fieldName ? The difficulty here is that is an array of array parentAssociationMappings. Creating a getParentAssociationMappingsFieldNames seems weird to me.

Should we create something like an FieldMappingAdapter and a AssociationMappingAdapter with methods

• getFieldName(array $mapping)
• getIdentifier(array $mapping)
• getFieldType(array $mapping)
• getModelClassName(array $mapping)
  Then
  A FieldDescription will expose a getAssociationMappingAdapter and won't expose a getModelClassName because you'll call $fieldDescription->getAssociationMappingAdapter->getModelClassName($fieldDescription->getAssociationMapping()).

I maybe can implements this for the next major, since I started something here sonata-project/SonataDatagridBundle#234

WDYT ? Do you have any idea to improve the situation ?

[phansys]

@phansys If you want to fully enforce the agnosticism around the model manager, we have to do something about the following lines

I agree, but the intention of my PR is not so ambitious. I just wanted to be Doctrine model agnostic, replacing any reference to entity (ORM), document (ODM), etc.

I think our next goal should be exactly what you're proposing. In some private projects I'm using this package to handle models from ReST APIs or things like that, but given these limitations I found myself making some nasty tricks in order to make it work.

WDYT ? Do you have any idea to improve the situation ?

I've to analyze our API again in order to determine which paths I can identify currently, but IMO the approach you've delineated is in the right way.