Backport /reassign quick command

The /reassign quick command works even when no multiple assignees are allowed of
there isn't any assignee yet. So for consistency, it's also be backported to
CE. But it functions the same as the /assign quick action.

app/services/quick_actions/interpret_service.rb

@@ -150,6 +150,24 @@ module QuickActions
       end
     end
 
+    desc do
+      "Change assignee#{'(s)' if issuable.allows_multiple_assignees?}"
+    end
+    explanation do |users|
+      users = issuable.allows_multiple_assignees? ? users : users.take(1)
+      "Change #{'assignee'.pluralize(users.size)} to #{users.map(&:to_reference).to_sentence}."
+    end
+    params do
+      issuable.allows_multiple_assignees? ? '@user1 @user2' : '@user'
+    end
+    condition do
+      issuable.persisted? &&
+        current_user.can?(:"admin_#{issuable.to_ability_name}", project)
+    end
+    command :reassign do |unassign_param|
+      @updates[:assignee_ids] = extract_users(unassign_param).map(&:id)
+    end
+
     desc 'Set milestone'
     explanation do |milestone|
       "Sets the milestone to #{milestone.to_reference}." if milestone

spec/services/quick_actions/interpret_service_spec.rb

@@ -362,7 +362,7 @@ describe QuickActions::InterpretService, services: true do
         it 'fetches assignee and populates assignee_id if content contains /assign' do
           _, updates = service.execute(content, issue)
 
-          expect(updates).to eq(assignee_ids: [developer.id])
+          expect(updates[:assignee_ids]).to match_array([developer.id])
         end
       end
 
@@ -431,6 +431,22 @@ describe QuickActions::InterpretService, services: true do
       end
     end
 
+    context 'reassign command' do
+      let(:content) { '/reassign' }
+
+      context 'Issue' do
+        it 'reassigns user if content contains /reassign @user' do
+          user = create(:user)
+
+          issue.update(assignee_ids: [developer.id])
+
+          _, updates = service.execute("/reassign @#{user.username}", issue)
+
+          expect(updates).to eq(assignee_ids: [user.id])
+        end
+      end
+    end
+
     it_behaves_like 'milestone command' do
       let(:content) { "/milestone %#{milestone.title}" }
       let(:issuable) { issue }