Review splitting and merging of activities

Created by: ederag

According to the help pages, https://github.com/projecthamster/hamster/blob/8326504b4a96b4cbc798b67efec27b3fdbbe83d1/help/C/merge.page#L11-L14

This does not work this way now. (the end part is simply disregarded) The relevant code is still there: https://github.com/projecthamster/hamster/blob/8326504b4a96b4cbc798b67efec27b3fdbbe83d1/src/hamster/storage/db.py#L454-L456 Need to investigate...