make delete event button red

When deleting a single event, the confirmation button is colored grey, same as cancel button. When deleting an event from a series, that confirmation button is colored red.

This MR will fix the missing red color for the delete single event button. Result:

*Note: I know this could be done better by using a single button and having the color set conditional like this: *

color: event.parentId ? theme.palette.normal.**unknown** : theme.palette.normal.negative

My problem is, I don't know the unknown color. Not setting a color there does result in a black/white background button. That is why I did use this rather bulky double button approach. Suggestions welcome.