Parent: #384

After the repository renaming I think we should change env var names to follow the new names.

And not only the Index index-backend repo was renamed to index, but the abbreviation for some env vars was changed from IDX to INDEX.

We should finish the renaming completely:

• TORRUST_INDEX_ -> OK
• TORRUST_INDEX_BACK_CORS_PERMISSIVE -> TORRUST_INDEX_CORS_PERMISSIVE or TORRUST_INDEX_API_CORS_PERMISSIVE
• TORRUST_IDX_ -> TORRUST_INDEX_
• TORRUST_IDX_BACK_ -> TORRUST_INDEX

cc @da2ce7 @mario-nt