fix repeater for simple timestamps

[tarnung]

Repeaters on simple timestamps are broken.
When the todo state changes, the last repeat property is created, a log entry is written and the planning item in the redux state is updated. The part in the title or description that represents the timestamp is not touched though, so while the item moves in the agenda, the change is not represented in text and is not persisted.

This fix connects the timestamps in title and description with their representation in planning items by keeping the id instead of generating a new one. It fixes the issue. I'm quite sure but not 100% certain that having planning items with the same id as parts of title or description won't cause problems. It makes sense, since the items with the same id represent the same thing. If it's preferred I could instead put a timestampId on the planning items and let them have their own unique id.

[munen]

I've added a reducer test case for it which fails on master, but works on this branch. I've also confirmed functionality manually.

What's left is to fix the eslint warning:

[tarnung]

I added support for deleting timestamps.
In the process I found a bug in removePlanningItem. When the timestamp editor is opened by clicking on a timestamp that does not belong to the currently selected header, removePlanningItem tries to remove the item from the wrong header.

[munen]

Great job, again 👍

I'll add it to the changelog and will merge it in!