Remove acl9, replace with SimpleRole

acl9 gem is a little bit opaque, had (at least one) rails 4.1.x incompatibility, and its tests are written in a convluted style with Test::Unit. Since our needs are simple (see what I did there?), rewrite, as SimpleRole, only implementing the methods that we use, and hardcoding (would be simple to generalize, but there is no need) class names for associations.
@yozlet, your comment here got nuked when i fixed the line above, but i'm :+1: to changing the name. should i do `grant_role` or `grant_role!` (with or without the :exclamation:?)