WIP: Refactor study description

This WIP pull request is breaking up the study description handler in multiple UI modules, as well as breaking up the study handlers file to be hosted on multiple files for easy edition.

I still need to decompose the post of the study handler in multiple post-only handlers that will help to maintain the page.

Feel free to make any comment if you feel that there is something really wrong.
Note that `inv_type` is an input parameter. I need to modify it if and only if it is `Other` or `None Selected`. For any other value, it is correct, so we just return it. This code was duplicated in 2 different spots previously (and it actually had a bug), so I wrapped the functionality in a function so we avoid code duplication.