Channel-value method arguments are in the wrong order

[dgw]

Back in #65, implementation of the .gh-repo command swapped the channel and key arguments to set_channel_value and friends. Since this plugin consistently uses the incorrect order, this hasn't been a problem; it just looks weird if one examines the DB directly.

Still, this should probably get fixed with some form of automatic migration. We can set a flag using plugin_values to indicate whether the migration has been run, too, now that such a feature exists in core.