Limit attributes of Dimension entity to stage and locale

[niklasnatter]

Right now, every DimensionContent is associated to exactly one Dimension. The idea behind this architecture is to make the Dimension class overwritable/extendable. This way, if somebody adds a targetGroup attribute to the Dimension class, it is available for every ContentRichEntity without any additional configuration.

Unfortunately, adding additional properties to the Dimension class has complex implications that we have not anticipated. The services for modifying the content need to be aware of the attributes of the dimension to process them correctly. For example, when publishing content, we want to publish the DimensionContent for a specific locale but all target groups. This means that adding an additional targetGroup requires to change the logic inside the PublishTransitionSubscriber

Furthermore, the possibility of adding additional attributes to the Dimension class makes a lot of the code of the bundle very abstract/vague/generic and therefore harder to understand. If we would decide to allow only a stage and a locale attribute, we could simplify a lot of the code and also our interfaces.

[wachterjohannes]

Related to #84

[niklasnatter]

Closing this as the Dimension entity was removed in #174

Furthermore, as explained in #84 (comment), I do not think that we should limit the dimensionAttributes anymore.