Add "This field is optional" below the labels of every optional field

[BlancaTortajada]

At the moment there isn't a way to know which fields are optional (you don't need to complete in order to publish). We can add this as the first line of the help text of the optional fields. We should also add it beneath the row header on the page before publishing (in regular font weight and using the color we use for help text).

[maxf]

@BlancaTortajada is this across all forms, or just title/description/additional info ?