Whenver sources are present, there should be exactly one default source.
Prior to this commit, it was possible to add the first source as a
non-default source and circumvent this invariant. This unfortunately
requires a bit of special-casing logic in s.Add and requires a query to
fetch the number of sources currently persisted.
Tests have been added to cover this as well as deleting the final
source, to ensure that there aren't any odd indexing panics that might
result from trying to find another candidate default source when
deleting.
493befbdc7