Reduce sql queries for groupname tests.

Conflicts:
	cms/djangoapps/auth/authz.py
Use mongo indices for all queries
STUD-1039
loc_mapper memoize all translations
STUD-1048
Location map uses a separate cache and defaults to general
Document performance optimization and auth generality changes.
Merge pull request #1954 from edx/dhm/loc_mapper_memoize

loc mapper memoize