Allow overriding of the 50 user limit in owner dropdowns.

As a performance enhancement for sites with misconfigured or
intentionally large and complicated owner lists, we automatically punt
from Dropdown to Autocomplete if we find more than 50 possible owners.

This allows you to write a trivial callback to customize the limit.
Merge branch '4.2/owner-autocomplete-switch-callback' into 4.2-trunk