jobset: Add a new "channels" tab.

It's very similar to "jobs" and the code is pretty much the same, except that we don't do filtering on it. At least it doesn't waste space for a filter option when there are usually WAY less channel jobs than ordinary jobs.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

Created by  aszlig  on April 14, 2015
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