[TASK] Silently deprecate flux:form.content

This ViewHelper and component is no longer necessary. In order to allow a smooth deprecation, the ViewHelper is left in place but will override the name and label of the `flux:grid.column` it is placed inside. This allows the `flux:grid.column` to be used by all subsequent parts of Flux to read content area names/labels.

Unfortunate side-effect: all LLL labels currently under `flux.$formId.areas.$contentName` must be renamed to `flux.$formId.columns.$contentName`. Page column labels completely unaffected.