remove firebase type from app

## Use Case

All firebase types should be hidden behind the image dataset interface but one has leaked out: the `dateCreated` in state/image-dataset/types.ts is a `Timestamp` which is a firebase type.

## Solution

Change the runtime definition of dateCreated to be a regular Date or number... and update the firebase dataset implementation to convert the value.

## Alternatives

Have firebase return a number instead of a Timestamp?