Unify S3 functions

Right now we have 3 functions for loading weights from s3:
* `vision/brainscore_vision/model_helpers/s3.py:load_folder_from_s3`
* `vision/brainscore_vision/model_helpers/s3.py:load_weights_file`
* `vision/brainscore_vision/model_helpers/__init__.py:download_weights`

To create a standard approach for this, we will:
(1) keep `load_folder_from_s3` and `load_file_from_s3` in `vision/brainscore_vision/utils/s3.py` as base functions
(2) potentially change `load_weights_file` into a generic wrapper function if needed, or combine into either of the other two functions
(3) create wrappers in `model_helpers/__init__.py` and `data_helpers/__init__.py` that set some default params (i.e. bucket name, etc) and handle plugin-specific needs; delete `s3.py` files in `model_helpers` and `data_helpers`