refactor: Remove legacy social plugin, PDP, product grid and WYSIWYG code

[loan-laux]

Resolves #5393
Impact: major
Type: performance|refactor

Issue

First of all, the Operator UI still has a Social section, which used to serve the purpose of adding social "share" buttons on the PDP, back in the 1.x days. This is not used anymore and this feature being present in Catalyst may confuse newcomers.

Second, lots of unused code relative to the legacy PDP, the product grid and the old WYSIWYG admin is still in this repo.

Solution

We need to remove the unused parts, which will help reduce the client bundle size, improving both Catalyst and customer login UI loading times by a little bit. Not to mention a small gain in DX as well.

Breaking changes

• The social plugin is no more.
• The following legacy components were removed:
  • productDetail
  • ProductField
  • ProductTags
  • ProductMetadata
  • PriceRange
  • VariantList
  • SocialContainer
  • VariantListContainer
  • ProductNotFound
• In the default-theme plugin, all the CSS files under the products directory were removed, with the exception of products/variantForm.less which still has literally 3 rules that are still being used.

Testing

1. Open the Operator UI
2. Notice that everything works just as it used to
3. Notice that the client bundle is a bit lighter

Before:

After:

[kieckhafer]

Thanks @loan-laux! Can you please take care of the DCO issues before I dive into this: https://github.com/reactioncommerce/reaction/runs/181723922

Thanks!

[machikoyasuda]

Actions to take after this PR is merged:

• Update the docs to remove any instructions for the Social plugin
• Update the docs to remove any old PDP/WYSIWIG component docs

[kieckhafer]

Looks good overall! Thanks for all of this @loan-laux!

One comment here about one last LESS issue, and then the DCO fix, and this should be good to merge.

[loan-laux]

@kieckhafer Removed the variantForm.less file and fixed DCO.