Add settings panel

[cleverbeagle]

Plays into Pupgrades. It’d be good if there was a built in pattern for managing settings as a user. Some sort of simple toggle switch type of UI and a data model on the user.

[cleverbeagle]

This works in tandem with #149 for a real nice integration.

[cleverbeagle]

This is a twofer. There needs to be a user settings panel but it'd also be helpful to have one for the app admins as well. Can group this with #151.

[cleverbeagle]

Thinking it would be better if this was handled in a separate collection. A data model like this should work well:

userId // ID of the user this settings object belongs to.
settings: {} // Blackbox object supporting key/value pairs.

To make this a bit easier to use, it'd be good to have a module like userSettings('sendMarketingEmails'). that automatically resolves to that field in the user's settings object.