Add "release" filter to merge request search page

This commit adds a new filter - "release" - to the merge request search
page. This filter takes a tag name (i.e. v1.2) as a parameter and
appends a URL parameter to the search query like "release_tag=v1.2".

app/assets/stylesheets/framework/filters.scss

@@ -249,7 +249,7 @@
 }
 
 .filtered-search-input-dropdown-menu {
-  max-height: $dropdown-max-height;
+  max-height: $dropdown-max-height-lg;
   max-width: 280px;
   overflow: auto;
 

app/controllers/projects/merge_requests_controller.rb

@@ -23,6 +23,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
 
   before_action do
     push_frontend_feature_flag(:vue_issuable_sidebar, @project.group)
+    push_frontend_feature_flag(:release_search_filter, @project)
   end
 
   around_action :allow_gitaly_ref_name_caching, only: [:index, :show, :discussions]

changelogs/unreleased/nfriend-add-releases-filter-for-merge-requests.yml

+---
+title: Add "release" filter to merge request search page
+merge_request: 19315
+author:
+type: added