Provide EE backports for filtering by approver feature

...ts/filtered_search/add_extra_tokens_for_merge_requests.js

...avascripts/filtered_search/available_dropdown_mappings.js

...ripts/filtered_search/filtered_search_dropdown_manager.js

...javascripts/filtered_search/filtered_search_token_keys.js

...ascripts/filtered_search/filtered_search_visual_tokens.js

app/assets/javascripts/filtered_search/visual_token_value.js

...ssets/javascripts/pages/dashboard/merge_requests/index.js

app/assets/javascripts/pages/groups/merge_requests/index.js

.../javascripts/pages/projects/merge_requests/index/index.js

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

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

lib/api/helpers/custom_validators.rb

lib/api/merge_requests.rb

...pts/filtered_search/filtered_search_visual_tokens_spec.js

spec/javascripts/filtered_search/visual_token_value_spec.js

spec/lib/api/helpers/custom_validators_spec.rb