Add more information to releases

[mattiaverga]

Since PDC is being retired and Bodhi is soon to become the source of truth for release info, I think it would be nice to have some more info:

• pre-beta/beta status: I think this state could be computed on the fly looking at the fxx.status in the config file
• release date: this would need to add a column to the Release model, the release date could be set automatically the first time the release status is changed to current

Do we need anything else to replace PDC?