Resolve empty gallery preview on demo post content

[aduth]

This pull request seeks to resolve an issue where the demo post content does not display the gallery preview correctly despite markup containing images. This is also the related cause of #1784. It appears attributes was removed in 118db19, but we must be able to parse the initial attributes from saved content. Currently we're only assigning new attributes for a block added during the editor session.

I'm not sure restoring the previous parsing is necessarily the correct solution given 118db19, but we will need attributes initialization.

Testing instructions:

Verify that the preview is shown correctly for the gallery block on Gutenberg > Demo.

[mkaz]

This change breaks opening up the gallery during editing and having the images selected in the media library (#1445) Could we just update the demo post content to include the proper comment attributes?

[aduth]

Could we just update the demo post content to include the proper comment attributes?

Sure. I wasn't familiar with whether this was an issue with the demo post content or the attributes parsing itself.

[aduth]

@mkaz In 8a71aef, I updated the serialized content to include images in the comment attributes. I'm finding that we're saving alot of data in images, trimmed down in my commit, but perhaps worth exploring separately (do we need all thumbnail sizes to be serialized, for example?).

[mtias]

@aduth @mkaz we should put some more thinking on this. I am not sure what attributes are necessary and for what reasons, but we should be minimal. If needed, we can discuss with the Media group.