Reduce MyGists::Search complexity by refactor #tags and #gists methods

[simeonwillbanks]

• Add MyGists::Search::Scope base class
  • Defines an interface for MyGists::Search::Scope::Tags and MyGists::Search::Scope::Gists
• Add MyGists::Search::Scope::Tags to define all possible ActiveRecord scopes used to search tags
• Add MyGists::Search::Scope::Gists to define all possible ActiveRecord scopes used to search gists
• Delegate most of MyGists::Search #tags and #gists to new MyGists::Search::Scope objects
• Replace single quotes with double quotes