add bottom type of box

Problem

Beekeepers in vertical beehives have a "bottom board". Sometimes its detachable and used to count fallen mites.

It would be nice to be able to add it in UI to reflect the state. It should be separate from "entrance" aka gate type.

Suggested changes

• design how bottom board should look like.

• add it to web-app as a new box type

• add it to DB as new box type using a migration

• add it on backend/graphql API layer