Add ocis update channel

[TheOneRing]

7.0 Will remove support for ownCloud 10.
We will provide 6.x builds for an extended period, users will have to either manually install a 7.x client or select a special update channel.

• Reactivate update channel selection for the desktop client
• Add ocis channel
• Keep default stable

[mmattel]

This also needs a prominent note in the docs release notes!