go.tools/dashboard/app: only accept commits when given master key
go.tools/dashboard/app: only accept commits when given master key

R=dvyukov, rsc
CC=golang-dev
https://codereview.appspot.com/37790044

d0f/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,8limbs/gotools,bryanxu/go-zh.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,webfd/go-zh.tools,bryanxu/go-zh.tools,8limbs/gotools