Use an existing name on build.snapcraft.io

There isn’t a build.snapcraft.io category on the forum, so filing under “other”.

We’re in the process of moving Mir and Co. code from Launchpad to Github, so we wanted to use build.snapcraft.io to build our snaps.

I’m added as collaborator on them, but the store just tells me the name is already taken:

Am I doing something wrong? Is it possible to use a pre-existing name that you don’t own (but are a collaborator for) build.snapcraft.io?

CC: @evan

This is #843.