Limits interaction with standby servers

* Non-GET/HEAD requests to standby servers are rejected with 403 Forbidden
* Standby jsguis displays a big warning.
* Warning should appear/disappear as the status of a server changes.
* Home tab lists servers and their status.

Comments on wording of various messages welcome, the one in server-not-ha-master.html in particular.

Doesn't attempt to redirect or link to the master server yet.