Refactor of Environments table and subitem components

Cleanup the environments table and the subitem components which are becoming unnecessarily complex.

This should include:

• breaking up large files, especially app/assets/javascripts/environments/components/environment_item.vue (700 lines long 😬)

It could also look into:

• replacing the table with the GlTable shared component
• modernizing some of the code (using Vuex, etc.)

The refactor would make bugs like #28937 less likely. Not urgent.

cc @ogolowinski