Add board membership read endpoint (#2799)

* Add board membership read endpoint

* Paginate board membership read endpoint

app/controllers/boards/accesses_controller.rb

@@ -0,0 +1,14 @@
+class Boards::AccessesController < ApplicationController
+  include BoardScoped
+
+  def index
+    set_page_and_extract_portion_from @board.account.users.active.alphabetically.includes(:identity)
+  end
+
+  private
+    def involvement_by_user
+      @involvement_by_user ||= @board.accesses.where(user_id: @page.records.map(&:id)).pluck(:user_id, :involvement).to_h
+    end
+
+    helper_method :involvement_by_user
+end

app/views/boards/accesses/index.json.jbuilder

@@ -0,0 +1,8 @@
+json.board_id @board.id
+json.all_access @board.all_access?
+
+json.users @page.records do |user|
+  json.partial! "users/user", user: user
+  json.has_access involvement_by_user.key?(user.id)
+  json.involvement involvement_by_user[user.id]
+end