Adding in snippet search functionality

app/controllers/search_controller.rb

app/helpers/application_helper.rb

app/models/snippet.rb

app/services/search/snippet_service.rb

app/views/layouts/_search.html.haml

app/views/search/_filter.html.haml

app/views/search/_results.html.haml

app/views/search/_snippet_filter.html.haml

app/views/search/results/_snippet_blob.html.haml

app/views/search/results/_snippet_title.html.haml

app/views/search/show.html.haml

lib/gitlab/snippet_search_results.rb