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

Pay special attention to database references, since this PR has a migration that can remove or change units and repo_content_units.

This pull request uses a small change in platform:

    https://github.com/pulp/pulp/pull/1382
minor update, this could be rewritten as 
```python
unit_key['checksumtype'] = verification.sanitize_checksum_type(user_checksum_type)
```
Just to save a call to `sanitize_checksum_type()` if `checksum_type` is `None` to begin with