Migrations: Move Fedora changes to new migration

Fedora-related changes should not be in the released 0.4.0 migration, but should be in their own. This is a holdover from when Fedora was supposed to be in 0.4.0.

Note that because Package.origin_pipeline was made NULLable, we cannot migrate backwards from 0005_fedora to 0004_v0_4_0. Suggestions for an actual default value welcome.
I don't generally care about `str` vs `unicode` in this context, it's just nice to minimize the diff so, e.g., `git blame` doesn't get thrown off in the future.