Hotfix/s3 keys

# Purpose
* Fixes #1236 
- The bug, being unable to access certain buckets, was caused by mishandling folders and more specifically lack thereof.

# Changes:
* The view for getting the hgrid list no longer uses any auxiliary methods and properly sorts out files and folders.
* build_urls now uses url_for functions

# Possible side effects:
* There is now much less information returned with hgrid data.
    - size, last modified, etc. are no longer included