Deleted the continue button on the RMA index

Deleted the continue button on the RMA index.
It does not have any function there.
Fixes #4891
Lock jquery-rails to 3.1.0 as 3.1.1 breaks data-confirm. Fixes #4892
remove 2-3-stable gem dependencies

since spree got bumped to 2.4.0.beta

Fixes #4893
Info for products with multiple variants

Setting SKUs for products with multiple variants can be confusing for novice users.

This adds a note on the product form in the place where the SKU field would normally appear. It lists several variant SKUs and invites the user to manage product variants.

Fixes #4897