Refactor issues filter in API

What does this MR do?

Ruse the IssueFinder for finding and filtering issues in the API. We cannot use the label filter of the issue finder because it matches only if all labels matches. The current API however matches if any of the label matches. I propose to change this in 9.0 since the /groups/:id/issues already uses the issue finder for that.

lib/api/issues.rb

@@ -5,28 +5,18 @@ module API
     before { authenticate! }
 
     helpers do
-      def filter_issues_state(issues, state)
-        case state
-        when 'opened' then issues.opened
-        when 'closed' then issues.closed
-        else issues
-        end
-      end
-
+      # TODO: Remove in 9.0 and switch to IssueFinder-based label filtering
       def filter_issues_labels(issues, labels)
         issues.includes(:labels).where('labels.title' => labels.split(','))
       end
 
-      def filter_issues_milestone(issues, milestone)
-        issues.includes(:milestone).where('milestones.title' => milestone)
-      end
-
       params :issues_params do
         optional :labels, type: String, desc: 'Comma-separated list of label names'
         optional :order_by, type: String, values: %w[created_at updated_at], default: 'created_at',
                             desc: 'Return issues ordered by `created_at` or `updated_at` fields.'
         optional :sort, type: String, values: %w[asc desc], default: 'desc',
                         desc: 'Return issues sorted in `asc` or `desc` order.'
+        optional :milestone, type: String, desc: 'Return issues for a specific milestone'
         use :pagination
       end
 
@@ -50,8 +40,7 @@ module API
         use :issues_params
       end
       get do
-        issues = current_user.issues.inc_notes_with_associations
-        issues = filter_issues_state(issues, params[:state])
+        issues = IssuesFinder.new(current_user, scope: 'all', author_id: current_user.id, state: params[:state]).execute.inc_notes_with_associations
         issues = filter_issues_labels(issues, params[:labels]) unless params[:labels].nil?
         issues = issues.reorder(params[:order_by] => params[:sort])
 
@@ -99,16 +88,14 @@ module API
         use :issues_params
       end
       get ":id/issues" do
-        issues = IssuesFinder.new(current_user, project_id: user_project.id).execute.inc_notes_with_associations
-        issues = filter_issues_state(issues, params[:state])
+        issues = IssuesFinder.new(current_user,
+                                  project_id: user_project.id,
+                                  state: params[:state],
+                                  milestone_title: params[:milestone]).execute.inc_notes_with_associations
         issues = filter_issues_labels(issues, params[:labels]) unless params[:labels].nil?
         issues = filter_by_iid(issues, params[:iid]) unless params[:iid].nil?
-
-        unless params[:milestone].nil?
-          issues = filter_issues_milestone(issues, params[:milestone])
-        end
-
         issues = issues.reorder(params[:order_by] => params[:sort])
+
         present paginate(issues), with: Entities::Issue, current_user: current_user, project: user_project
       end