Changes to show tag category only once for each base class.

Added changes to store value of TAG_CLASSES array into a hash and then use invert on it in order to use each model only once to add tag categories for them, having base class more than once in that array was causing duplicate entries in the pull down.

https://bugzilla.redhat.com/show_bug.cgi?id=1160209

@dclarizio please review & test.
1. Should the inverted hash just be another constant, much like we did with some of the other constants?  In fact, looking at the usage of TAG_CLASSES, it should just be a Hash in the first place.  Perhaps we can just change it to that and change the 1 or 2 callers?
2. If we can't do 1, this can be done simpler with:

```ruby
TAG_CLASSES_FOR_MODEL = Hash[TAG_CLASSES.each_slice.to_a]
```