Coverage/adapters

[Snowflake8]

The aim is to improve coverage. It does this using the mocks for ListSaleActivity and ListBookActivity It delects SingletonHolder Adds a few tests for utils (like DatabaseException)

Pull reviewers stats

Stats for the last 180 days:		User	Total reviews	Median time to review	Total comments
	so7fie	29	13h 38m))	60
	Snowflake8	26	8h 39m))	55
	Huntsekkern	30	6h 15m))	54
	RaulinN	29	7h 28m))	38
	DGoedtkindtEPFL	34	15h 26m))	30
	Laniakea1999	18	12h 8m))	14