Fixes to action execution and Transmission module

Created by: SunilMohanAdapa

This has been running the background for me quite a while. I decided to finally submit this but @jvalleroy's pull just before I make this request. I believe there is place for both the modules (as a general FreedomBox policy on multiple apps providing same service).

This modules introduces Transmission as daemon for downloading torrents.

Before that it fixes some issues with actions.py and updates that tests.