Add ability to set a layout per taxonomy

[edalzell]

Closes #2003

[jasonvarga]

Can you make it the same as Collection?

The layout method falls back to layout, and it's not necessary to set the fallback in TaxonomiesStore.

cms/src/Entries/Collection.php

Lines 406 to 414 in 2fd7d5b

	public function layout($layout = null)
	{
	return $this
	->fluentlyGetOrSet('layout')
	->getter(function ($layout) {
	return $layout ?? 'layout';
	})
	->args(func_get_args());
	}

cms/src/Stache/Stores/CollectionsStore.php

Line 49 in 2fd7d5b

->layout(array_get($data, 'layout'))

[jasonvarga]

Actually, this is unfortunately inconsistent with collections.

On a collection, you can define template and layout in the yaml, which will set them for the entries.

On a taxonomy, the template method is what will be used for the automatically routed index url.
The layout method is what's used for that route too.

This PR is allowing you to adjust the layout for the automatically routed taxonomy page. Not the entries. The issue #2003 is about people wanting the term layouts to be customizable.

This is another thing that would be resolved if we scrapped the automatic routing as per statamic/ideas#839