Tag organization & tagging constraints

[selfcommit]

This issue is being presented as an opportunity for the Github community to make any further suggestions about implementation before an attempt is made to implement a solution.

Work will start on or about July 13

Details from the Simian feature request doc are below:

Problem:
Tags are currently a bit difficult to manage.

Tagging currently does not respect database constraints (Generating a tagged computer can happen multiple times in a single tag)

Solution:
Create a better hierarchy for tag management.
Revise the tagging data model