Add staging changes verification process

In order to prevent the situations with changes being deployed to production without verification on staging, for example like

1. MR_1 is merged
2. MR_1 is deployed to staging
3. MR_1 is not directly tested on staging but also not deployed to prod
4. MR_2 is merged
5. MR_2 is deployed to staging
6. MR_2 is tested, during testing a login bug appears that's not present in prod. Since no one is initially aware that MR_1 is also not yet deployed to prod there's a lot of confusion on what caused the bug. See Internal Slack Thread

this MR proposed a solution to use slack deployment messages to indicate when changes were tested