In Ruby 5.2, we get a warning when using the "RANDOM()" function:
DEPRECATION WARNING: Dangerous query method (method whose arguments are
used as raw SQL) called with non-attribute argument(s): "RANDOM()".
Non-attribute arguments will be disallowed in Rails 6.0. This method
should not be called with user-provided values, such as request
parameters or model attributes. Known-safe values can be passed by
wrapping them in Arel.sql().
This warning doesn't make much sense, though, since RANDOM() is a common
function which is not dangerous at all. However, since the warning is
annoying, we'll probably have to find a way to deal with it.
So I'm extracting all our RANDOM() usages into a method. This way we'll
only have to change one method to avoid this warning.
I've chosen `sample` because it's similar to Ruby's Array#sample, and
because `order_by_random` would be confusing if we consider we already
have a method called `sort_by_random`.
