Ensure properties/keys cannot contain '.' character

[thclark]

Since the data structure in values can be deeply nested, it is extremely helpful to be able to define a path to that node in the json schema.

Thus, restricting to keys which do not contain the '.' character allows us to define a path as "input_values_schema.prop_a.prop_b" and refer to that node of the schema independently.

This is extremely useful in e.g. creating drag and drop type schema editors.

[thclark]

Worth also noting that many frontend solutions for managing twines and json schema will assume this constraint already, and would break without it.