Add maturity column to uploads table

[preaction]

The distribution's maturity is encoded in the release's filename. The CPAN::DistnameInfo does this correctly, deciding if the release is "released" or "developer".

We should add a column, maturity, to the uploads table. This column should allow either the value released, for a distribution that is installable from CPAN, or developer for a release that was not indexed by CPAN (We don't call released stable because we are not making a value judgement).

Once we have this column, we can start thinking about removing columns from other tables that are tracking the distribution maturity (including "release_data.distmat").