Editor: Media: support custom media sizes defined by site specific code

[designsimply]

Raised by @jkudish

The Editor should support custom media sizes defined by sites (via add_image_size)

Before we can add custom thumbnail sizes in the Calypso editor, support for providing this information will need to be added to the REST API. (478-gh-io)

[designsimply]

#bug-scrub #pre-gutenburg-bug-purge

Tested and confirmed that the Calypso Editor still does not support custom media sizes using the following steps:

1. Go to wp-admin > Settings > Media.
2. Update "Thumbnail size" to 50 x 50.
3. Go to Posts > Add New, select an image, change the size to Thumbnail in the Attachment Display Settings, and click "Insert into post."
4. Go to My Sites > Add and select (+) Add > Media in the editor toolbar.
5. Select and insert an image, click the image, and click the "-" button in the image toolbar until the image can no longer get any smaller.

Result: the image inserted using the wp-admin editor respects the custom media thumbnail size set in step 2 while the image inserted using the Calypso editor does not.

^{Seen at https://wordpress.com/post/design5279.wordpress.com/11665 using Firefox 57.0.1 on macOS 10.13.2.}

^{Seen at https://design5279.wordpress.com/wp-admin/post-new.php using Firefox 57.0.1 on macOS 10.13.2.}

[creativecoder]

With the Calypso editor deprecated, I don't think this issue is relevant any longer, but please feel free to re-open it if that's incorrect.