List merge fields for a board

Returns the merge-field catalog the template editors offer for this board, grouped as the Meteor useMergeFields hook groups it: recipient, board, sender, showing, lease, application, general, company (the hook's system and companyFields groups are named general and company here, and its per-board locations/aptlets groups are one flat board list, since the path already names the board).

recipient is the contact field list plus the company's Person custom fields; board is the board's mergeable fields plus the image-embed, card-detail, contact-table and card-link entries.