Refactor the way that focus-mode configuration is represented
in state and whittle down the API (selectors and actions) a little
for clarity.

Reduce the object depth for the focus-mode state. Use a single
function to configure focus mode. Concede that user-focus is the
only focus mode for now, and as such valid focus-mode configuration
must contain a (valid) user object.

Note: There is still some lingering inconsistency in the user
identifiers provided by the LMS app. Canvas speedgrader uses app
settings to configure focus mode (`js-config` object) and sets
the `username` property of the `user` object to a username-
formatted value (i.e. a username, no authority indicated). Non-
Canvas graders do not configure focus mode via the settings,
but instead dispatch RPC messages. These pass a `username`
property whose value is formatted like a userId
(`acct:<username>@authority>`) but it still provided via
the `username` property.

AFAIK, nobody/no service sends a `userid` property in the
focus-mode user object, but we document that we do support
it so I've left it extant until we figure this out.