Support periodic alarm notifications for additional channels

As an SRE, I want to be reminded on active alarms in configurable interval, so that I do not forget about alarms when a bigger number of alarms has been generated and I am distracted with other tasks.

Right now this feature is only available for Webhooks, e.g. to control traffic lights.

We should offer it for any kind of notification channel. Obviously it makes sense also for chat boards since Slack, HipChat etc. do not have a concept for tracking _unread_ messages.

# Implementation

There are some checks in the API code and notification to ensure that periodic notifications are only used with Webhooks. They need to be replaced by safeguarding measures to limit the notification frequency for the new supported channels.

It is worth also to look at the '60-seconds-notifications' topic and whether it is affected by our change. In general, one minute should be an adequate resolution to control the period.

# Acceptance Criteria

* Periodic notifications shall be enabled for any notification channel
* The minimal notification interval shall be configurable per notification channel in the API config