Merge branch Mario/status_effects (L merge request)

This has been near the top of my design files for a while, as it is a stepping stone to unifying several separate systems. Initial design is implemented here to allow future extensions.

Status effects is a system that handles buffs, debuffs and general effects applied to any entity. Networking to clients is handled in a similar manner to the inventory system, meaning support for non-player entities is server side only at this stage.
In its current form this system is handling the burning effect along with both powerups and buffs (all of which are not fully ported to it yet).
As a side bonus, partial support for a mapper-side buff timer is included, in the buffs_finished field.