Move OC_Channel to system config

The Nextcloud and ownCloud updaters allow someone to configure a custom release channel, this can then be used to publish different versions. (e.g. one channel stays on 9.x while another one already gets 10.x)

There is however one big problem with it: The value is effectively stored in the app config, which is stored in the database. So to be able to read the update channel a connection to the database is necessary. This is quite error prone and also causes some of the issues in the original ownCloud updater.

This moves the channel registration to the config.php and also includes a repair step.