Change how users delete entries from the database

Add a button to delete a row from the data table. This deletion can be reverted until the changes are saved to the database. Because we changed the way entries are deleted, a popup now asks user to confirms that they want to save changes to the database.

This merge request only changes the way the user deletes an entry from the database. It does not fix any problem covered by #189 (closed).

Implement part of #196 (closed) because some of the functionalities there is required for this merge request.