Add filtering of test summary data by Perl maturity

[preaction]

It would be nice to allow filtering of the test summary data (/v3/summary) by Perl maturity so we can get only reports from stable or development Perls (default to all Perls).

We should add two special values to the perl query parameter:

• stable will select only stable Perl versions
• dev will select only development Perl versions

Specifying one of these values requires joining in the perl_versions table (cpan-testers/cpantesters-schema#13).

[preaction]

I couldn't add these to the existing perl variable, so I created a new perl_maturity option instead.