Update date renderer to transform string values into numbers

[draperd]

In order to be able to easily compare dates values we need to convert string values in and out of the field into numbers. We should do this in both the default native renderer and also the Atlaskit renderer.

Make sure to add unit tests for this.

[draperd]

I've added some more tests for this using a custom renderer to ensure that the capability works but I have not yet updated any of the packaged renderers to automatically convert date strings to numbers, etc.