Add a skip_users filter to the project users API

This functionality is available in the /autocomplete users pseudo-API.
We're attempting to replace that with the canonical API, so it needs
support for this parameter too.

doc/api/projects.md

@@ -855,6 +855,7 @@ GET /projects/:id/users
 | Attribute | Type | Required | Description |
 | --------- | ---- | -------- | ----------- |
 | `search` | string | no | Search for specific users |
+| `skip_users` | array[int] | no | Filter out users with the specified IDs |
 
 ```json
 [

lib/api/projects.rb

@@ -489,11 +489,13 @@ module API
       end
       params do
         optional :search, type: String, desc: 'Return list of users matching the search criteria'
+        optional :skip_users, type: Array[Integer], desc: 'Filter out users with the specified IDs'
         use :pagination
       end
       get ':id/users' do
         users = DeclarativePolicy.subject_scope { user_project.team.users }
         users = users.search(params[:search]) if params[:search].present?
+        users = users.where_not_in(params[:skip_users]) if params[:skip_users].present?
 
         present paginate(users), with: Entities::UserBasic
       end

spec/requests/api/projects_spec.rb

@@ -1494,6 +1494,17 @@ describe API::Projects do
 
         expect(response).to have_gitlab_http_status(404)
       end
+
+      it 'filters out users listed in skip_users' do
+        other_user = create(:user)
+        project.team.add_developer(other_user)
+
+        get api("/projects/#{project.id}/users?skip_users=#{user.id}", user)
+
+        expect(response).to have_gitlab_http_status(200)
+        expect(json_response.size).to eq(1)
+        expect(json_response[0]['id']).to eq(other_user.id)
+      end
     end
   end