Refactor securesync

depends on #1533 .  More major refactoring, this one was more painful to extract.

Moved `securesync/users` to a new app, `facility`.

Good time to do it now, with the major refactoring of `shared`, the low test coverage for `release-0.11`,  the interns likely moving to use KA Lite next week, and another upcoming bug bash.

@aronasorman please take a look after completing #1533 .  Glad to walk through any parts that might be confusing.