Add sizes attribute for responsive images markup

[mor10]

Addresses #50.

Prototype. Currently only works properly for "regular" width images.

Notes:

• $GLOBALS['block_width'] section needs work as the theme uses fluid widths for all images on wider screens. Current values are effectively random.
• To provide correct sizes attributes for alignwide and alignfull images we need a way of knowing, from within the wp_calculate_image_sizes filter, what the width display setting is for the current image. At present it is only defined in a class on the parent <figure> element which to my knowledge is inaccessible from the filter.

Works as prototype for testing of Gutenberg issue 6177.

cc @azaozz

Todo:

• sizes attributes for alignwide and alignfull
• sizes attribute for featured images

[mor10]

sizes attribute for featured images added and merged in #493.

[mor10]

Closing in favor of #629