Cache the own_device, to avoid (innumerable) queries for this object 

(which, after install, never changes)

For logging in, for example, this reduces queries from ~40 to ~25.

Testing:
* Tested fresh install (works)
* Ran test suite (no unexpected failures)

PR Help:
* Timing: 24 hours.  This is a small change, with appropriate testing.
* @jamalex could you code review the code change?
* @aronasorman could you code review the code change