Set width and height for Block Editor images

[t-wright]

Description

Fixes #1391 by adding the correct width and height attributes to images added to post content using the block editor. Needs proper testing and consideration. cc @brandon-m-skinner: as mentioned in BB8-6850.

Checklist

Please make sure the items below have been covered before requesting a review:

• This change works and has been tested locally (or has an appropriate fallback).
• This change works and has been tested on a Go sandbox.
• This change has relevant unit tests (if applicable).
• This change has relevant documentation additions / updates (if applicable).

Steps to Test

Outline the steps to test and verify the PR here.

1. Enable the block editor with add_filter( 'use_block_editor_for_post', '__return_true' );
2. Create a new post
3. Add an image block, choosing an image from the media library that has dimensions wider than a large image
4. In the block inspector, choose a display size other than Full Size
5. Publish the post and observe that the width and height attributes on the <img> element are those of the full size image.
6. Apply this PR
7. Reload the browser and observe that the dimensions reflect those of the size chosen in the block attributes.

[rinatkhaziev]

@t-wright Looks good, but I didn't get to test it. Do you mind fixing the lint issues, as pointed out here?

[rinatkhaziev]

@t-wright one follow-up request - could you please leave comments inside those functions signifying which parts exactly were modified?

[mikeyarce]

I was testing a few potential solutions and then noticed this has been fixed in Core for 5.7 here: https://core.trac.wordpress.org/ticket/51865

Closing this PR as the core fix addresses this issue.