Add priority field to the import form for queue entries

[David Planella]

When approving an entry in the imports queue (e.g. https://translations.edge.launchpad.net/+imports/2647986) it would be very useful to be able to set up the priority for the template, which right now is not available there and gets set as 0.

Setting it afterwards is a bit of a pain, since you have to remember the template you've exported, navigate to it and open the +admin form to set it

After the initial effort of setting up template priorities for Ubuntu packages, I'd like to assign the priority value to each template we approve, and having a "Priority" field in the import form would make this a lot easier.