Feature request: Make json_settings_url in AppParameters user configurable

[yschroeder]

I'd like to configure the repository where the settings come from. Currently it is hard-coded to the software: https://github.com/prusa3d/PrusaControl/blob/master/parameters.py#L222

My use case is the following: We have a Prusa i3 MK2S at work and everyone is allowed to use it (Slic3r runs on their own computers). However, as we print mostly mechanical objects I tuned our settings. I distribute these settings by having a git repo that is cloned by each user into ~/.Slicer. As PrusaControl already has the feature to update print settings on startup, it would be awesome if it would instead pull from our settings, instead of the official ones.

Maybe those hard-coded parameters (also the ones following after json_settings_url should be moved to data/defaults.cfg?