Create a channel repository sync schedule manager in the GUI

[phibid]

To manage the channel repository sync in the GUI, we have to go on every channel and from there, schedule by hand the repo sync schedule.

This is a little bit fastidious to go one by one if you add lots of channels at once. And there is a risk to forget 1 repository schedule as we don't have an overall view in the GUI of the syncs (this only exists via the spacecmd API call softwarechannel_listsyncschedule).

So I think that it could be good to have a main page to manage all the channel repo sync.