Avoid automatically generating translation keys that override default ones

As previously discussed in WEBUI-62, given that a user is managing custom configuration in Studio, under no circumstances should default translation keys be overridden by keys generated automatically in Studio.

Suggested approach: Have Web UI's default messages.json file available for each target platform, so that when a translation is automatically added we can introduce a check to verify if that key already exists by default or not. If it exists, the new key is not added. If it doesn't exist, the new key is added.

Things to consider:

• The automatic creation of labels is managed at the Translations feature builder level.
• There's already a check in place to avoid adding the same key if it already exists in the custom translation file. All there is to do (at first glance) is to also check if it exists in the default one.
• Since we can add multiple labels at the same time, it would be great if we could read the default translation file just once (bonus points!).
• Ricardo is already loading the default translation file for the preview => check if we can take advantage of that.