various code and test fixes for UserPrefs.js

Specific changes:
* renamed prefs page URL to prefs.html
* added load/savePlayerInfo to dummy_responder and to responder tests
* fixed a bug in which loadPlayerInfo was returning a bunch of extraneous info, including passwords
* added prefs.html to main navbar so people can browse there
* cleaned up syntax/logic of UserPrefs.js to match other JS modules
* added QUnit tests for UserPrefs.js

I believe we can now say this fixes #101