[Enhancement] Allow alternate feed sorting

[tedwardd]

Right now neonmodem sorts by "new". This is fine, and probably necessary when viewing multiple systems at once, but it would be nice if we could detect (or hard code) the available sorting options and allow the use of them when viewing a specific system.

[mrusme]

Each system could offer a map of sorting options, where the string key can be harmonized across different systems, and the value would be a string that is specific to the system, e.g. ["new"] = "latest", ["new"] = "newest", ["most-recent"] = "most_recent_replied_to", ...

The sorting selector for "All systems" would only show the options that have the same key available across multiple systems (e.g. "new"). This way e.g. "most-recent" would not show up for "All systems" but only when one specific system was selected. Alternatively "most-recent" could be introduced to other systems by using either of the available sortings, e.g. "newest", meaning it wouldn't actually sort by most recently commented, but make the option available nevertheless.