unify S3 buckets

we currently use a variety of buckets (`brainio-brainscore`, `brainscore-vision`, `brainio-unittests`, ...). Let's unify this all into the `brainscore-vision` bucket with an appropriate folder structure. The folder structure is already somewhat in place (e.g. top-level `data/` and `models/` folders) and we should organize all data by plugin directory.

Start by searching the repository for the listed bucket names and existing top-level data/ and models/ paths, then trace how plugin directories access them. Done means the buckets are unified under brainscore-vision with an appropriate plugin-based folder structure and no remaining use of the old bucket names.

Written by the indexing model from the issue text.