Add a configuration option to set a cap for total items per user

[rghedin]

• [x] I have read this document: https://miniflux.app/opinionated.html#feature-request

It would be nice a configuration option to limit total items a user can have at anytime.

On instances with multiple users, unintentional abuses are likely and affects all other users. For instance, a single user that adds several high frequency feeds can reach six digits items. I’m not a programmer or system admin, but I imagine this affects instance performance.

A cap on maximum items a user can have (summing read and unread items) would mitigate this issue without collateral damages — what are the odds that someone with a +100k items backlog will read all of them within the default window for unread messages, 180 days?