添加离线页面，在Discourse重建或启动时显示

[jomaxro (Joshua Rosenfeld)]

@amotl

Right now, we just went for the online upgrade through /admin/upgrade and found Discourse did not go offline at all!

Discourse should never go offline when running upgrades through Docker Manager (/admin/upgrade). Does it usually go offline for you? If so, we should start a support topic about that.