Apturl feature to add (whitelisted) repositories

Apturl currently does not allow adding new repositories for security reasons. This limits its usefulness, there are certain repositories that show high quality and good QA. So we should add a whitelist feature to apturl so that we can add trusted repositories. The criteria/policy that such a repository must follow is discussed in the separate blueprint "partner-repositories".

The implementation will use the "channel" mechanism of gnome-app-install/app-install-data-partner and a new key is added to the apturl URL spec named "channel=value" that denotes to the channel that should be added (if not already there). Steps to implement:
1. add "channel=value" to apturl parser and GUI
2. rename "app-install-data-commercial" to app-install-data-partner
3. put the trusted repo sources.list and signing keys into app-install-data-partner