Ensure usernames are case insensitive

[dstufft]

This is a current issue on PyPI. Usernames are case sensitive so I can have dstufft, Dstufft, etc.

We need to both make sure this cannot happen in Warehouse, and figure out what sort of transition plan we will use.

[alex]

I think http://www.postgresql.org/docs/9.0/static/citext.html is probably the right way to do those, you might need to add some support to sqlalchemy.

[dstufft]

Yea I think using citext is probably the right way. Will need to figure out how to handle the duplicate name issue on PyPI before PyPI can be migrated.