[WIP] 1165355 - Sanitize checksum types.

SHA and SHA-1 are the same checksum algorithm, but Pulp considered them
to be separate algorithms. This commit ensures that Pulp no longer
stores the SHA-1 algorithm as "sha" in the database, and it also
contains a migration that resolves existing "sha" objects.

https://bugzilla.redhat.com/show_bug.cgi?id=1165355

This pull request is a work in progress. There are no unit tests yet, and there are still other types that need to be addressed with the migration. I wanted to submit it now so that we can get some more eyes on it while I work to complete it. Please take a look and let me know if you can think of any issues that this introduces, or if you know of any other problems that this doesn't address. Pay special attention to database references, since this PR has a migration that can remove or change units and repo_content_units.