Refactor the PluginController

DRY approach on the blocks that call the "updateVersionMetadata"
service.
Changed the controller object to improve the legibility of code.
Remove frontend stuff

Deleted assets folder and all frontend tasks. All of them are
unnecessary because this project is an api. The frontend will be an
Ember App completely decoupled.
Remove old TagController on policies.js

The TagController was removed on 9b137b6 but I forgot to remove it from
the policies file.
Remove `date` attr. on PluginInstallation Model

Replaced by the existing `createdAt` attribute, provided by
Waterline ORM.
Refactor the EmberOk Response

Now uses the global config to enable/disable sideloading.
Refactor controllers to accept Ember.js Requests

Ember Data sends JSON objects. This commit changes all req.params to
emberUtils.parseValues, from "mphasize/sails-ember-blueprints".