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

Send out a Notification upon creation of a new Request entity. The receivers are specified by the corresponding Role logic this Role Request is for. Currently Organization and Club Administrators will receive "new request" messages about respectively Mentor and Club Membership requests. Fixes Issue 442.