Refs #34323 - sanitize setting fixtures

Setting fixtures should not be used to override defaults for tests unless meant for every test.
If we mean to override the setting value for all tests, we should override value and not the default.

Fixes #34323 - use defaults from memory

This loads values from database only if the values are in the database.
If there is no value populated for the Setting, we want to read the default from memory.

Refs #34323 - explicit value for Rails 6.1

In c693cd9 we have worked around an issue for distinguishing between value explicitly set by user and not set.
This workaround doesn't work properly in Rails 6.1 and we should improve it to work properly.