Improve performance of triggered chat commands

When the trigger endpoint is called, it has to find the right service
for the given project. However, the old implementation did much more.

For example, it build a list of the missing services on this project.
This whole process took about 750ms _each time_. The current
implementation is expected to perform 10x better, as it only searches in
the current projects services. Given the service has to be configured
anyway, this can be done.

changelogs/unreleased/zj-slow-service-fetch.yml

+---
+title: Improve performance of slash commands
+merge_request: 8876
+author:

lib/api/services.rb

@@ -661,6 +661,14 @@ module API
     end
 
     trigger_services.each do |service_slug, settings|
+      helpers do
+        def chat_command_service(project, service_slug, params)
+          project.services.active.where(template: false).find do |service|
+            service.try(:token) == params[:token] && service.to_param == service_slug.underscore
+          end
+        end
+      end
+
       params do
         requires :id, type: String, desc: 'The ID of a project'
       end
@@ -679,9 +687,8 @@ module API
           # This is not accurate, but done to prevent leakage of the project names
           not_found!('Service') unless project
 
-          service = project.find_or_initialize_service(service_slug.underscore)
-
-          result = service.try(:active?) && service.try(:trigger, params)
+          service = chat_command_service(project, service_slug, params)
+          result = service.try(:trigger, params)
 
           if result
             status result[:status] || 200