Assign unique ids to boards, columns, and cards

[dev-guy]

Blocks #66

Ids can be used to store objects in a database. For example, there is currently no way to relate a database row in a Card table to a card on a Kanban board.

Changes Needed

1. Add id prop to and generate a new one (uuid) if missing
2. Card objects in the Svelte store will have a new id attribute (string) that can be specified via the cards that are passed on via a <Kanban> prop. Generate new ids (as uuids) when not user-specified.
3. The HTML elements for cards already have id attributes in the form of card-#-col-#. This can be reworked to look up the card (and hence its id) in the store since $column provides column-card lookup. Then use the card ids as HTML element ids.
4. Repeat 2 and 3 for columns
5. Pass board, column, and card ids in svelte component event data where applicable