Add self-service update and delete for board member management

Add more self-service support for board member management, specifically update and delete operations.

Note: Board Actions that are still awaiting signature need to be completed (or aborted) before involved board members can be updated.

Board actions refactor to populate signatures from signable_document_signatures instead of board_members. This also means removing the soft delete board member functionality as it is no longer necessary to keep track of deleted board members linked to completed board actions. Create db migration to remove the status column from the board_members table.