Remove time stub in test teardown

ActiveSupport::Testing::TimeHelpers does not include auto removal of
this type of stubs in test teardowns for now.
Show maturities in project header
Remove references of old reward attributes
Remove explicit call to Reward#to_s in views
Send happens_at attribute in spec creating reward

It's a required attribute
Remove GROUP BY clause from Discover class

Follows removal of categories from the JOIN.