WIP: Media library thumbnails

[owenhoskins]

WIP for #946

• Implementation of pica.toCanvas to generate thumbnails on the fly in MediaLibrary/CardImage.js
• Using react-intersection-observer in EditorWidgets/GalleryPreview to throttle the toCanvas calls
• Using react-virtualized in the MediaLibrary to handle large sets of data and also throttle toCanvas calls

[verythorough]

Deploy preview for netlify-cms-www ready!

Built with commit 56b9856

https://deploy-preview-1230--netlify-cms-www.netlify.com

[verythorough]

Deploy preview for cms-demo ready!

Built with commit 56b9856

https://deploy-preview-1230--cms-demo.netlify.com

[ro-savage]

@owenhoskins - Will this allow users to upload and previews images without them being directly uploaded to github master branch?

[erquhart]

No, just performance improvements for the media library interface.

_{Sent with GitHawk}

[mbergeronupgrade]

Waiting for this mostly to fix #787

[tech4him1]

@owenhoskins Let me know if you want any help on this! I'd be glad to work on the backend integration (saving thumbnails) part if you're ready for that.

[owenhoskins]

@tech4him1 So glad you asked, that'd be awesome! I can hit you up on Gitter tomorrow or sometime this week? I finally got clear of a few things so this is coming back in focus!

[erquhart]

@owenhoskins going to go ahead and close this as stale, but please comment to re-open if you dig any further on it!