CSV dataset file improvements

When creating a CSV dataset, you can now upload multiple files simultaneously. Previously, you had to upload your first file and then navigate to the Data tab within your dataset to append new files.

Each CSV must have the same headers and data types, but now you can upload them all at once. This feature is particularly beneficial for large datasets that are divided across multiple files.

Additionally, you can upload multiple files at once in the Data tab of your dataset. Other overall user experience issues have also been addressed in this UI.