SearchController#show should be refactored

SearchController#show currently does a lot of things, in an ugly way:

    if params[:project_id].present?
      @project = Project.find_by(id: params[:project_id])
      @project = nil unless can?(current_user, :download_code, @project)
    end

    if params[:group_id].present?
      @group = Group.find_by(id: params[:group_id])
      @group = nil unless can?(current_user, :read_group, @group)
    end

These are permission controls that could be done directly in the search services.

---

    @search_results =
      if @project
        unless %w(blobs notes issues merge_requests milestones wiki_blobs
                  commits).include?(@scope)
          @scope = 'blobs'
        end

        Search::ProjectService.new(@project, current_user, params).execute
      elsif @show_snippets
        unless %w(snippet_blobs snippet_titles).include?(@scope)
          @scope = 'snippet_blobs'
        end

        Search::SnippetService.new(current_user, params).execute
      else
        unless %w(projects issues merge_requests milestones).include?(@scope)
          @scope = 'projects'
        end
        Search::GlobalService.new(current_user, params).execute
      end

1. The search service selection should be handled by a SearchService.
2. The default scope instantiation could be done directly in the search services.