Change `destination_name` to `destination_slug` for bulk imports API

There has been confusion about what the destination_name field in bulk imports refers to. It actually refers to the destination slug (i.e. the last part of the URL). However, because it includes "name" and is also referred to as "name" in the UI, users confuse it with the group name.

We therefore need to change any reference to destination_name to destination_slug. This will require several steps:

Implementation suggestions:

Suggestion 1:

• add a destination_slug column to the bulk_import_entities table on the database
• populate each row's destination_slug with the data from the destination_name column
• change any occurrence of destination_slug in the bulk_importer and api to destination_path, for both Groups and Projects
• delete the destination_name column
• update the api documentation (both REST and GraphQL)
• update the UI (see #347510) to clarify that the input field is for a slug not a name

Suggestion 2:

Add destination_slug on API level without changing it on database level. This should provide clarity for the user that entered information should be a group slug, not a group name.