Add support for default values

[Krever]

Default values defined in json schema or scala code can be used to pre-fill the form on creation.

Initially we could support only json schema, depending how complicated it is to get default values from scala in typeclass derivation.

[keynmol]

depending how complicated it is to get default values from scala in typeclass derivation.

There is a well known and pretty horrible but copypastable way of doing it