Add filtered search to MR page

app/assets/javascripts/dispatcher.js.es6

app/assets/javascripts/filtered_search/dropdown_hint.js.es6

...vascripts/filtered_search/filtered_search_dropdown.js.es6

...s/filtered_search/filtered_search_dropdown_manager.js.es6

...avascripts/filtered_search/filtered_search_manager.js.es6

app/assets/javascripts/search_autocomplete.js.es6

app/controllers/projects/merge_requests_controller.rb

app/views/projects/merge_requests/index.html.haml

app/views/shared/issuable/_search_bar.html.haml

changelogs/unreleased/add-filtered-search-to-mr.yml

features/project/merge_requests.feature

spec/features/issues/filtered_search/dropdown_label_spec.rb

spec/features/issues/filtered_search/filter_issues_spec.rb

spec/features/merge_requests/filter_by_labels_spec.rb

spec/features/merge_requests/filter_by_milestone_spec.rb

spec/features/merge_requests/filter_merge_requests_spec.rb

spec/features/merge_requests/reset_filters_spec.rb

spec/features/search_spec.rb

spec/javascripts/search_autocomplete_spec.js

spec/support/filtered_search_helpers.rb