Maximum user limit

This adds a configurable maximum user limit. To enable it, set `maximumUsers` in the default or a provided config file.

This also adds a `GET /accountBuffer` route which, if `maximumUsers` is set, will return the number of accounts left until the limit is hit. It does not divulge the user count.

This still needs tests which I will add tomorrow and manual testing in the example apps.

Closes #286