Allow candy admin to display initial message on login form

Builds on PR #303, adds core option `initialMessage` which is displayed at top of login form when user first presented with it.  This message will be replaced by 'login failed' as appropriate.

Intended for things like link to Ts & Cs and/or special information relating to fixed domains (PR #303).
It's intended for `termsMessage`, which presumably contains links to the terms of service or instructions for using Candy (or whatever).  If you think it worthwhile, I could change that selector to `#chat-modal-body`.  (Or I can move it to my local customisations).