Add "Maintenance Status" badge API

[Turbo87]

Related to #2436 and #2437, we want to offer crate owners the option to add a badge to their README files that reflects the current maintenance status.

We currently use https://shields.io/ to display the maintenance status badge and the service seems stable enough for now to keep using it. I would propose adding an API endpoint that reads the maintenance status for the selected crate from the API, and then redirects to the corresponding shields.io URL like:

https://img.shields.io/badge/maintenance-actively--developed-brightgreen.svg