Building on app creation

[stuartpb]

If a Git repo already exists when creating an app, we'll build it.

This is the right thing to do anyway (for flows like ones where a user creates/pushes a repo then creates an app), but it's also necessary for retrieving apps (plushu/plushu-apps#5, plushu/plushu-git#9).

[stuartpb]

As noted in plushu/plushu-git#9, this should only happen when there isn't an existing build (since create-app can be called multiple times during the life of an app, eg. on every push when git-receive-init and app-repo-binding are installed).