Reader: add improved spam reporting process

[designsimply]

Raised by @bluefuton

Spam reports in the past have often been low-quality. It was suggested that offering a modal dialog where users can tell us more about their spam complaint would help:

This will require a new design for the modal, and database changes along with a new REST API endpoint to capture the reports.

[stale]

This issue has been marked as stale because it hasn't been updated in a while. It will be closed in a week. If you would like it to remain open, can you please comment below and see what you can do to get things moving with this issue? Thanks! 🙏