Refactor ActsAsTaggableOn::Tag.public_tags and Use Kaminari

Refactor SQL Query for getting public tags and replace will_paginate with kaminari

* Using EXPLAIN, refactor ActsAsTaggableOn::Tag.public_tags to reduce cost from "18.18..18.35" to "14.99..15.15"
* Replace will_paginate with kaminari since kaminari:
  * does not clobber the ActsAsTaggableOn::Tag.public_tags query
  * does not pollute any objects like ActiveRecord::Base
  * has a cleaner UI
* Remove WillPaginate initializer
* Generate kaminari config
* Generate bootstrap kaminari theme
* Refactor decorators to delegate pagination methods to kaminari
* Refactor MyGists::Search to delegate kaminari pagination methods to scope
* Refactor MyGists::Public.tags to use kaminari
* In the views, replace will_paginate helper with kaminari helper