Make Excuses removable

Currently there is afaik no real way to remove an excuses from an absence. I think there should be a good way, especially in the person overview.

assigned to @ZugBahnHof

added part::frontend prio::2 type::feature labels

added 1 deleted label

added board::ready label and removed 1 deleted label

added board::todo label and removed board::ready label

mentioned in merge request !111 (merged)

added board::ready label and removed board::todo label

closed with merge request !111 (merged)

mentioned in commit 35c5bfc7