Adds to Melange a tags framework based on taggable-mixin.
The taggable-mixin allowed only tag per Datastore model. This is extended
framework allows any arbitrary number of tags per Datastore model. Also,
now one can define different models for different Tag types which are all
inherited from the base Tag model provided by taggable-mixin.
The GHOPTask model makes use of 2 tags per model, one for difficulty and the
other for task_type, both using the tags framework.
Reviewed by: Paweł Sołyga
(extracted verbatim from the end of coverage.py)
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