Store does not allow publishing a snap in the public store when the same name is used in another store for another series

[Simon Fels]

We're about to publish two snaps

 * network-manager
 * bluez

Both are already published for 15.04 in a customer-specific store.

Now we want to publish the snaps with the same name but for series 16 in the public Ubuntu store. According to Natalia this should be possible but the store interface doesn't allow this currently.

For a bit more background: We're going to convert the 15.04 device to series 16 soon but until then we have to keep the 15.04 customer specific store with the snaps around. Future customer specific snaps will get a suffix when uploaded to the customer store.