Implement debian:suite collection

https://freexian-team.pages.debian.net/debusine/reference/collections.html#category-debian-suite specifies the behaviour of a debian:suite collection. Now that the core collection data model and management machinery has landed, we need to implement this to support #277 (closed).

We do not yet need to implement publishing (i.e. Packages/Sources/Release file generation, URL layout, etc.); just enough to represent an external repository in debusine's database.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information