feat(image widget): media library gallery tools (#6087)

[demshy]

Summary

When using a media library with multiple images option, the user had to replace all images in case they wanted to change the selection in any way (except reorder). The idea was, to give them the ability to remove only certain images, add extra to the selection or replace certain images.
The button wording also needed an update to match the use case.

Closes #6087

Test plan

Usage video
https://user-images.githubusercontent.com/19568230/155301973-bf757da2-7d51-405c-b620-c45003c27057.mov)

I tested with both cloudinary and uploadcare.

config.yml used for testing (single image widget added to see that nothing changed there):

local_backend: true

backend:
  name: git-gateway
  branch: main
  squash_merges: true

display_url: #your-display-url
logo_url: /media/brand/logo.svg
media_folder: static/media/uploads
public_folder: /media/uploads

media_library:
  name: cloudinary
  config:
    cloud_name: #cloudinary-name
    api_key: #cloudinary-key

slug:
  encoding: ascii
  clean_accents: true

collections:
  - name: posts
    label: Posts
    label_singular: post
    folder: content/posts
    create: true
    slug: "{{slug}}"
    editor:
      preview: false
    fields: [
      {name: title, widget: string},
      {name: gallery, widget: image, i18n: duplicate, hint: Select at leat 3 images, required: false, media_library: {config: {multiple: true}}},
      {name: single, widget: image, i18n: duplicate, hint: Only one image, required: false, media_library: {config: {}}}
    ]

Checklist

Please add a x inside each checkbox:

• I have read the contribution guidelines.
• Code is formatted via running yarn format.
• Tests are passing via running yarn test.
• The status checks are successful (continuous integration). Those can be seen below.

A picture of a cute animal (not mandatory but encouraged)

[erezrokah]

Hi @demshy and sorry for the very late review, this is PERFECT 🚀

Only thing, that it would be good to add tests for this behavior, however since we don't have existing tests for it 😅 I don't think it's a blocker.

I'll wait for the GitHub actions maintenance to be over to make sure CI passes, so probably this will get released on Monday.

[demshy]

This is great new @erezrokah, thanks.
I will try to add tests the next time we get some new ideas for the widget and contact you guys on slack if I find myself struggling :)
Looking forward to future contributions.

[erezrokah]

This is shipped in netlify-cms@2.10.190 and netlify-cms-app@2.15.70.

Thank you for the contribution @demshy!