Provide a setting, "inhibitAdvancedLogin", and default to false.

We make it un-retained, so packagers can change it as part of the app
distribution, and have that change affect users when they get the new
version of the app.
.settings.get("aSetting").get("value") is awkward and leaves unnecessary
room for mysterious typos, and misleading exceptions if the setting name
is mistyped.  Add a .getValue("aSetting") which throws a ReferenceError
for names of absent settings, and provides a shortcut for getting, and
use it.

Implement some essential settings tests, to demonstrate proper behavior.
Merge pull request #320 from SpiderOak/issue-319-condition-advanced-login

Provide a setting, "inhibitAdvancedLogin", and default to false.