refactored database helper
token hash must be unique
added initial uniqueness integration tests;  currently these are order-dependent, suggesting a problem
removed token_hash unique constraint:  since token hashes are salted, it is highly unlikely that duplicate token strings would lead to duplicate token hashes; it is also difficult to test this constraint
resolved test-order-dependency issue by using mongoose model-level "remove", which empties the associated collection
Merge branch 'feature-uniqueness' into develop