Fix loader centering and scrolling on Team Members > Invitations tab

### Description

The Invitations tab under Team Members has two issues:

- The loading component is not centered in the middle of the page: update the layout so the loader is properly centered vertically and horizontally
- The entire datatable currently scrolls: update the layout so the table header remains fixed and only the inner table body scrolls

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate