title: limiting tags to use with ActsAsTaggableOn
body: <p>I'm currently evaluating whether to use ActsAsTaggableOn or roll my own.
  I can't have users create tags at random but want to define a list of allowed tags for a project model. Users associated with that project can only select the tags that are allowed for that project (through project and tag model where a project can have multiple tags).
  The sane thing to do seems to use ActsAsTaggableOn, but after googling for more than one hour I can't find a lot of documentation on my use-case.</p>