Add a list of sick services in sick notifications

[Eijebong]

Right now when a service is sick we get something like (on slack and zulip at least):

Status changed to: sick.

Status: Sick
Time: 09:37:14 UTC+00:00

It would be nice to have a list of sick services just like in the dead notification.

[valeriansaliou]

Noted! Open to PR. Note that in the case it goes from DEAD to SICK in my use case I do not want to have a list of services provided, so that should be configurable and disabled by default.