Move "Maintenance Status" into the crates.io UI

[Turbo87]

Currently the "Maintenance Status" of a crate/version is hardcoded in the Cargo.toml file. If a crate is no longer maintained it is easy for this status to become outdated though, since a new release would be needed to update the status. We are aiming to simplify setting this status for the maintainer, but keeping it as a field on the crate resource directly in the API, and exposed in the crates.io dashboard for the maintainers to change.

What follows is the rough list of tasks that need to be done to achieve this goal:

• add the maintenance_status column to the crates database table
  • initialize the column from the current value in the Cargo.toml of the corresponding crate?
• expose the maintenance_status field in the API
• support updating the maintenance_status field using the API for crate owners
• show the maintenance_status value in the crates details page sidebar
• add a user interface to the dashboard to update the value for each owned crate

[kornelski]

Maybe the status could be stored in the registry? It could be updated similarly to the yanked field.