New electricity effect

This MR replaces smoke with an electricity effect on enemies hit by a player holding the Disability buff.

The effect is implemented using the lightning effect of the Arc. Internally the Disabled status effect has been renamed to Stunned and moved from the Buffs code to the Status Effects code.

Closes #2717 (closed)

Note that this buff is currently disabled by default, it can be enabled with g_buffs_disability 1.

This effect can be quickly tested with:

sv_cheats 1
join
give buff disability

Old version of the electricity effect implemented as model:

Video test: https://www.youtube.com/watch?v=MYock1MO68w