KCard: Add prop validators for 'layout' and 'thumbnailDisplay'

[MisRob]

Summary

This is one of the follow-ups to #625

1. Use constants for all allowed layout and thumbnailDisplay prop values
2. Add custom prop validators for these two props so that KCard throws an error when an invalid value is passed.

Guidance

You can see an example in KImg:

kolibri-design-system/lib/KImg/index.vue

Lines 56 to 60 in 6a6865f

	const ScaleTypes = {
	CENTER_INSIDE: 'centerInside',
	CONTAIN: 'contain',
	FIT_XY: 'fitXY',
	};

kolibri-design-system/lib/KImg/index.vue

Lines 56 to 60 in 6a6865f

	const ScaleTypes = {
	CENTER_INSIDE: 'centerInside',
	CONTAIN: 'contain',
	FIT_XY: 'fitXY',
	};

kolibri-design-system/lib/KImg/index.vue

Lines 124 to 132 in 6a6865f

	/**
	* Specifies how an image should be scaled within the container.
	* Can be one of `'centerInside'`, `'contain'`, or `'fitXY'`.
	*/
	scaleType: {
	type: String,
	default: 'centerInside', // needs to be duplicated rather than using ScaleTypes.CENTER_INSIDE, otherwise it doesn't render correctly in the auto-generated Props documentation
	validator: isValidScaleType,
	},

[AllanOXDi]

completed in #707