If issues are filtered by multiple labels, then GROUP BY statement
is used to filter only issues with these labels. This is problem in
combination with '.includes' which may use JOIN statement to load
associated resources in single query instead of a separate query.

Then the generated SQL query fails because associated resources
are included in 'SELECT ...' statements but not in the GROUP BY
statement.

Usage of '.preload' instead of '.includes' ensures that Active Record
uses a separate query instead of JOIN.

Closes #4376