Store partially-filled in package form data as a draft.

[icmurray]

Allow a user to save a partially-complete and probably invalid package-create form in order to come back to it later.

We decided on blob storage over a state field on the package:

• prevents a history of the drafts being stored in the revision history. This is a particular problem if we start auto-saving, say every 20s.
• the blob can store arbitrary extra data, such as how far through the wizard we are.