Add option to archive project

[ziima]

What's the problem this feature will solve? Similarly to Github, it would allow the users to mark the projects as archived and no longer maintained. It could prevent later process of detecting whether the project is unmaintained.

The project would be still visible and usable, but clearly marked as archived and unmaintained.

Describe the solution you'd like Aside of Delete a project, a possibility to Archive the project.

Additional context There will most likely be discussion about the availability of the projects name. I suggest to allow take over the name on archived projects and only allow unarchive the project only if the name is still available.

[miketheman]

Triaging this with @ewdurbin and we came up with some implementation ideas:

• boolean flag on Project.is_archived (database migration)
• some way to store an archive message from the Owner (probably a new ObservationKind)
• UI for Project Owners to flip the bit and add a message
• UI element (warning yellow) on Project and Release pages that shows archived and message (similar to yanked?)
• Guard against further uploads if project.is_archived==True

If someone wanted to take a stab at this, we'd be happy to review.