Fix various numerical admincp options which did not step in the incremental values as intended, while allowing arbitrary values in the range to be entered
Implement XF2.2 LinkableInterface interface for various entities where it makes sense
Change 'threadmark-index-icon' to silently return an empty string is passed null for the threadmark index, and allow the href parameter to work without being in update mode
Add "Use threadmark index icon instead of avatar" per-forum option (default: false)