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

 * 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