Issue 293 utf8 in share room passwords

Fixes #293, in a way that properly reconciles #288.

Also eliminate unnecessary non-basic multilingual plane character consolidation.

I've tested this manually with an issue intense password, one that includes plain ascii, spaces, url-sensitive ascii, latin1, and non-basic multilingual plane characters (simple Chinese and Hebrew).

(It's worth noting an obscure, surprising behavior associated with our implementation and SpiderOak's password-protected share room implementation:
* If any password-protected share room has the same password as your current login, and you are logged in by entering the credentials rather than by virtue of a restored auto-login, then the share room will be automatically authorized.
* The protected share room can be from any account, just so long as it has the same password.
* This will not happen if you're in a restored login session.  In that case the webview doesn't have the login credentials.
* This implicit authentication will probably be eliminated in refinements of the server's support for password-protected share rooms.)