Use ETag to improve performance of issue notes polling
Improve performance of polling for issue notes by using caching with ETag as described in #26926 (closed).
- This is very unlikely to make the performance worse.
- It should provide quite generic mechanism that can be easily applied to other endpoints
- It will be implemented only in the Rails app
- It is performed before authentication and authorization to avoid querying the database on cache hit at all
- Introduce new route
- Modify frontend code to pass
last_fetched_atas a request header