Add more custom labelX items to Measurable

[OAGr]

labelConditionals: optional(array(string))
labelStartAtDate: optional(dateTime)
labelEndAtDate: optional(dateTime)

These should validate with the following validations on the backend:
if there is a labelStartAtDate, there must be a labelEndAtDate.
if there is a labelEndAtDate, there must be a labelStartAtDate.
There cannot be both a labelOnDate and a labelStartAtDate/labelEndAtDate