Fix for #1237: allow users to set their own default language.

title as simple as it says.  

Design:
* Store `default_language` on each `FacilityUser`
* `default_language` is `None` by default, falling back to server-level default.
* `default_language` gets set when a `FacilityUser` selects a language in the dropdown.
* When the `FacilityUser` logs out and logs back in, the `default_language` should be used.

Testing:
* Test that setting the server default language by admin still works
* Test that setting the `FacilityUser`'s default language (via lower left dropdown) works both in the current session, and after logging out / logging back in.