Add Orbot (Tor-proxy) support

[spacecowboy]

Orbot: https://guardianproject.info/apps/orbot/

If anyone is interested in implementing this feature, open a PR with title WIP: Add Orbot support to show that you are working on it, then ask me anything in the PR if you need help. I'll be happy to suggest which code goes where.

Goal

Add Orbot support to Feeder. This would allow the app to download RSS feeds via the Tor-proxy.

Implementation details

• Add a global setting which enables/disables Orbot-integration.
• If setting is enabled, then all HTTP requests should be routed through Orbot. This includes RSS-sync and image loading.
• If setting is toggled, this should check if Orbot is installed and if not: prompt the user if he wishes to install it. If no is selected: toggle Orbot back to false.