Test fix and bulk actions

### Fix
Fixes the test for my latest PR: deletion should send a 204 HTTP status 

### Feature: bulk actions
Bulk actions are a way to process actions on multiple element of a folder at the same time. Currently, there is only move and remove actions are supported.

This closes #91.

Please note that the UI can be improved but I just got an idea a few minutes ago: I should have used the "upload status" bar.