Add a JSON cards endpoint for workflow columns (#2800)

* Add JSON endpoint for cards in workflow columns

* Add cards URL to column JSON responses

* Add coverage for column cards authorization

app/controllers/boards/columns/cards_controller.rb

@@ -0,0 +1,19 @@
+class Boards::Columns::CardsController < ApplicationController
+  include BoardScoped
+
+  before_action :set_column
+
+  def index
+    set_page_and_extract_portion_from @column.cards.active.latest.with_golden_first.preloaded
+    fresh_when etag: @page.records
+
+    respond_to do |format|
+      format.json
+    end
+  end
+
+  private
+    def set_column
+      @column = @board.columns.find(params[:column_id])
+    end
+end

app/views/boards/columns/cards/index.json.jbuilder

@@ -0,0 +1 @@
+json.array! @page.records, partial: "cards/card", as: :card

app/views/columns/_column.json.jbuilder

@@ -1,4 +1,5 @@
 json.cache! column do
   json.(column, :id, :name, :color)
   json.created_at column.created_at.utc
+  json.cards_url board_column_cards_url(column.board, column)
 end