Sticky sessions for ui-server

[lorenzo-cavazzi]

Originally from https://github.com/SwissDataScienceCenter/terraform-renku/issues/784

In short, it would be nice to have sticky sessions for the UI server for the HTTP requests so that requests from different tabs reach the same pod in the backend. In that way, we will always set up multiple WebSockets channels with the same backend pod, allowing us to prevent multiple queries to the same endpoints.