AWS slack notifications improvements

[nikolayilyin]

We need to reduce the total number of notifications or make them less annoying (reduce the amount of '@ here' tags) and improve some of them.

1. Add current month budget limit to the budget notification, i.e. "The month limit is X and currently we spent Y"
2. Send budget notification for each $5000 spent in a month.
3. Questionable improvement - don't send forecast of the budget, budget notification for each 5k might be enough.
4. Budget notifications should be the only one with '@ here' tag.
5. Personalize each IDLE notification, in case it is impossible to understand who is responsible for the instance - tag in the notification some preset persons with permissions to do something with the instance, for now let it be: @ Dima Openkov, @ Justin Pihony, @ Nikolay Ilin