Make preview breakpoints configurable

[florrie-90]

Adds functionality to pass breakpoints from config/twill.php to the Previewer.vue component. The breakpoints will be used as the available sizes for the preview modal.

'preview' => [
        'breakpoints' => [
            [
                'size' => 1280,
                'name' => 'preview-desktop',
            ],
            [
                'size' => 1024,
                'name' => 'preview-tablet-h',
            ],
            [
                'size' => 768,
                'name' => 'preview-tablet-v',
            ],
            [
                'size' => 320,
                'name' => 'preview-mobile',
            ]
        ]
    ],

Fixes #1601