go.tools/dashboard/app: reject /commit requests from old builders
go.tools/dashboard/app: reject /commit requests from old builders

This is so that we don't corrupt our commit history with reports
from old builders, after the migration to the latest build dashboard.

LGTM=dvyukov
R=dvyukov
CC=golang-codereviews
https://codereview.appspot.com/130300043

webfd/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,webfd/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,skyportsystems/go.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools,skyportsystems/go.tools,d0f/go-zh.tools,skyportsystems/go.tools